UNIVERSITY OF

SOUTHERN CALIFORNIAGEOPHYSICAL LABORATORY

DEPARTMENT OF GEOLOGICAL SCIENCES

A SEISMICITY STUDY FOR PORTIONS OF THELOS ANGELES BASIN, SANTA MONICA BASIN, AND

SANTA MONICA MOUNTAINS, CALIFORNIA

By

James Alexander Buika 1" and Ta-liang Teng

Technical Report No. 79-9

A Report on Research Project Supported by the U.S. Geological Survey

Los Angeles, California

September, 1979

|Now at Texaco, Inc., 3350 Wilshire Blvd., Los Angeles, Ca. 90010

EARTHQUAKE HAZARD RESEARCH IN THE LOS ANGELES BASIN AND ITS OFFSHORE AREA

Ta-liang Teng, James Alexander Buika Thomas L. Henyey, John K. McRaney

Kenneth E. Piper and Richard A. Strelitz

Department of Geological Sciences University of Southern California Los Angeles, California 90007

USGS CONTRACT NO. 14-08-0001-16704 Supported by the EARTHQUAKE HAZARDS REDUCTION PROGRAM

OPEN-FILE NO.81-295

U.S. Geological Survey OPEN FILE REPORTThis report was prepared under contract to the U.S. Geological Survey and has not been reviewed for conformity with USGS editorial standards and stratigraphic nomenclature. Opinions and conclusions expressed herein do not necessarily represent those of the USGS. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS....................................... ii

LIST OF ILLUSTRATIONS .................................. vi

LIST OF TABLES ......................................... ix

ABSTRACT............................................... X

INTRODUCTION........................................... 1

General Statement.................................. 1

The Study Area: Major Faults...................... 13

Southern California Seismic Network................ 17

Crustal Velocity Model............................. 25

Station Delays..................................... 32

PROCEDURE.............................................. 38

Data Collection.................................... 38

Data Processing.................................... 42

Data Editing and P Residuals....................... 43

RESULTS................................................ 49

LATEST FAULTING: PREVIOUS WORK........................ 74

Newport-1nglewood Fault Zone....................... 74

Palos Verdes Hills Fault........................... 76

iii

Page

Santa Monica Fault Zone: East of Beverly Hills.... 78

Sierra Madre Fault Zone............................ 82

Santa Monica Fault Zone: West of Beverly Hills.... 82

FAULT-PLANE SOLUTIONS.................................. 86

General Statement.................................. 86

Newport-Inglewood Fault Zone....................... 87

Fault Plane Solutions: Santa Monica Bay........... 98

Fault Plane Soutions: Santa Monica Fault, East

of Beverly Hills............................... 105

Fault Plane Solutions: Santa Monica Fault, West

of Beverly Hills............................... Ill

SEISMICITY PATTERNS, MAGNITUDES, AND FOCAL DEPTHS...... 114

STRESS REGIME AND PREFERRED FAULT-PLANE SOLUTIONS...... 122

CONCLUSIONS............................................ 129

REFERENCES............................................. 134

APPENDIX A: List of seismic stations used for

recording and relocating 1973-1976 earth­

quakes ............................................. 142

APPENDIX B: Improvement in earthquake relocations

by applying three successive iterations of the

station delay constant to seismic station data..... 151

APPENDIX C: HYP071(REVISED) data output summary

for three typical earthquakes...................... 153

IV

Page

APPENDIX D: Summary list of 423 earthquakes relocated

in the study area which have occurred between

January 1, 1973 and December 31, 1976.............. 160

APPENDIX E: Epicenter, magnitude, and focal depth

plots for the study area........................... 171

APPENDIX F: Epicenter, magnitude, and focal depth plots

for Point Mugu area................................ 176

APPENDIX G: Problems limiting the precision of a

fault-plane solution............................... 184

v

LIST OF ILLUSTRATIONS

Figure Page

1. Physiographic provinces of southern California.... 2

2. Bathymetric map of the Santa Monica basinand San Pedro basin. .............................. 4

3. Fault map showing all faults displaced sinceat least Quaternary time. ......................... 8

4. Geologic time scale used to classify faultactivity .......................................... H

5. Geologic map detailing proposed connections between the Hollywood fault (Santa Monica fault) and the Raymond Hill fault. ................ 15

6. Seismic stations monitored and maintainedby the CIT and USGS, as of January 1, 1977........ 18

7. Location map for the Baldwin Hills SeismicNetwork and the Long Beach Seismic Network. ....... 21

8. Plan view of areal extent for each crustal velocity survey conducted in southern Cali­ fornia ............................................ 27

9. Cross section of the six crustal velocitymodels ............................................ 29

10. Shaded region represents overlap of 1973- 1976 station data between this study and USGS study by Lee and others (1979) ............... 40

11. Flow diagram outlining editing procedure 44

VI

Figure Page

12. Epicenter plot for all 423 earthquakes re­ locating in the study area during 1973-1976....... 50

13. Magnitude plot of 1973-1976 earthquakes........... 53

14. Focal depth plot for 1973-1976 "A" and "B"quality earthquake relocations only............... 55

15. "A" and "B" quality earthquake relocations,1973-1976......................................... 58

16. Fault-plane solution map for individualearthquakes, 1973-1976............................ 61

17. Composite fault-plane solution map for earth­ quakes, 1973-1976................................. 64

18. Compressive stress pattern for the study area derived from 1973-1976 individual fault- plane solutions................................... 69

19. Compressive stress pattern for the studyarea derived from 1973-1976 composite fault- plane solutions................................... 71

20. Fault-plane solutions 1, 2, and A................. 88

21. Fault-plane solutions 3, B, and 4................. 90

22. Fault-plane solutions 5 and 6..................... 92

23. Fault-plane solutions C and D..................... 93

24. Fault-plane solutions E, 7, and 8................. 94

25. Fault-plane solutions 9, F, and 10................ 97

26. Fault-plane solutions G, H, I, and 11............. 99

27. Fault-plane solutions J and 12. ................... 102

28. Fault-plane solutions 13 and 14................... 104

29. Fault-plane solutions 15, 16, 17, and 18.......... 106

30. Fault-plane solutions 19, 20, and 21.............. 107

VI1

Figure Page

31. Fault-plane solutions 22, K, 23, and 24........... 110

32. Fault-plane solutions 25 and 26................... 113

33. Outline of seismicity trends...................... 115

Vlll

LIST OF TABLES

Table Page

1. Seismic network in the Los Angeles basin area...... 23

2. Crustal velocity structure. ........................ 25

3. Explanation of earthquake relocation quality rating and data output summary lists found in Appendices C and D .............................. 34

4. List of individual fault-plane solutions. .......... 63

5. List of composite fault-plane solutions. ........... 66

6. List of seismic stations used for recording andrelocating 1973-1975 earthquakes. .................. 143

7. List of seismic stations used for recording andrelocating 19 76 earthquakes ........................

Maximum variation between the strike and dip of possible fault planes due to changes in crustal velocity models. ...............................

IX

ABSTRACT

Data from seismic station networks in southern

California have been collated to relocate 423 earthquakes

occurring between January 1, 1973 and December 31, 1976,

within the Los Angeles basin, the Santa Monica basin, and

the Santa Monica Mountains. From this data set, the

seismicity pattern, fault-plane solutions, and compressional

stress vectors have been derived and are compared with the

most recent known fault displacements in order to determine

the casuative fault for some of these earthquakes, the

preferred orientation of fault planes and probable focal

mechanisms.

The Los Angeles basin region is presently responding

to a regional northeast-southwest-oriented compressional

stress regime. Earthquakes on the northwest-trending

faults of the Newport-Inglewood, Whittier, and Palos Verdes

Hills fault zones describe fault-plane solutions dominated

by reverse-slip mechanisms. Right-lateral strike-slip is

often a secondary component of motion. Focal mechanisms

support the concept of convergent right-lateral wrench

faulting occurring in the Los Angeles basin. x

Seismic activity is concentrated in the Los Angeles

and Santa Monica basins rather than within the Santa Monica

Mountain block. Most activity appears to terminate at

the Santa Monica fault system rather than continue to

the north, along the projected trend of the northwest-

trending Newport-Inglewood and Palos Verdes Hills fault

zones. The western portion of the east-northeast-trending

Santa Monica fault system was probably the causative fault

for the magnitude 5.9 Point Mugu earthquake of February 21,

1973. Besides reverse faulting, normal faulting appears

to be associated with the western portion of the Santa

Monica fault system. The eastern portion of the Santa

Monica fault, between Hollywood and Glendale has also been

the locus for a cluster of microseismicity. Reverse

faulting has dominated this eastern cluster. A minor

component of left-lateral strike-slip displacement is

possible in several fault-plane solutions along the Santa

Monica fault system.

XI

INTRODUCTION

General Statement

A seismicity study has been conducted for portions

of the Los Angeles basin, the Santa Monica basin, and

the Santa Monica Mountains (Figure 1). Bathymetric

features for the offshore portion of the study area are

shown in Figure 2. The Los Angeles basin, Santa Monica

basin, and Santa Monica Mountains are tectonically active

structures that have formed at the juxtaposition of three

physiographic provinces the Transverse Ranges, Peninsular

Ranges, and California Continental Borderland (Figure 1) .

These three provinces have a common point at the inter­

section of the Newport-Inglewood uplift and the Santa

Monica fault. The east-west-trending Transverse Ranges,

represented at its southern boundary by the Santa Monica

Mountains, transects the northwest-trending structural

grain associated with the Peninsular Ranges to the south,

the Coast Ranges to the north and the San Andreas fault

system. Consequently, a complex system of faults has

Figure 1. Physiographic provinces of southernCalifornia. Rectangle outlines study area. Geographical boundaries are 118000'W to 119 15 f W longitude and 33°45'N to 34°15'N latitude. The intersection of the Santa Monica fault and the Newport-Inglewood fault (dashed line) is shown. After Yerkes and others (1965) and Junger and Wagner (1977).

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been activated within the wedge of continental landmass

lying west of the San Andreas fault in response to a

regional NNE-SSW compressional stress regime.

All recorded earthquakes occurring from 1973 through

1976 have been compiled from data received at seismic

stations from all seismograph networks operating in

southern California and have been located using Lee's

and Lahr's (1975) computer program. Epicentral patterns,

fault-plane solutions, and compressional stress vectors

have been derived from the microearthquake data to deter­

mine the causative faults for some earthquakes, the pre­

ferred orientations of fault planes, and probably focal

mechanisms. These results are discussed in terms of

fault displacements and fault geometries.

The objective of this study is to add to the under­

standing of the contemporary tectonic environment within

the Los Angeles basin region by combining known geologic

and geomorphic data with recently derived seismic data

for this same region. If seismic activity can be asso­

ciated with a fault or fault zone, then it confirms

the active status of that fault. From this relationship,

the seismic record can be studied to understand the

present tectonic regime. Future earthquake rupture is

assumed to occur along pre-existing faults, with the

greatest probability of renewed movement on active and

potentially active faults, as defined below (Ziony and

others, 1973).

Just as the seismic record represents the active

tectonic regime, the geologic record represents the

past tectonic environment. Fault displacements, measured

in the field from geologic relationships, assist in iden­

tifying recency of movement along faults. This informa­

tion can be used to extend the historic and instrumental

seismicity records. Since surface rupture can be roughly

correlated with a magnitude 6 or greater earthquake

(Alien and others, 1965), the youngest preserved strati-

graphic or geomorphic evidence for faulting can be used

to demonstrate and bracket the time of the latest signi­

ficant seismic activity. This study compares known Qua­

ternary faulting in the Los Angeles basin region with the

1973-1976 earthquake data set to access similarities and

dissimilarities bewteen the present seismic record and

the Quaternary geologic record. Geologic and geomorphic

evidence, gathered from various published field studies,

has been used to document the most recent displacements

on known faults in the Los Angeles basin region.

Figure 3 is a fault map for the study area displaying

only known or inferred fault displacement of Quaternary to

present age. The map was compiled from several sources;

onland faults have been adopted from Ziony and others'

(1974) comprehensive compilation of recency of fault move­

ments along the southern California coastal area; off­

shore faults have been compiled from recently interpreted

7

Figure 3. Fault map showing all faults displaced since at least Quaternary time.

Legend

solid line (or star) - Holocene displacement (<11,000 y.)

long dashed line - Late Quaternary displacement (11,000 - 500,000 y.)

short dashed line - Quaternary displacement (500,000 - 2,000,000 y.)

dotted line (P) - Pliocene (>2,000,000)

star - historic earthquake

sub-bottom acoustic-reflection profiles by Vedder and

others (1974), Green and others (1975), Junger and

Wagner (1977), and Nardin and Henyey (1978).

Following Ziony and others (1974), faults demon­

strating displacement during the Holocene time (approxi­

mately 11,000 y. B.P. to present) have been classified as

"active" faults; faults demonstrating displacement since

at least Quaternary time (approximately 2-3 m.y. B.P.

to present) have been classified as "potentially active"

(Figure 4). These criteria have been followed in con­

structing the active fault map for the study area (Figure

3). The fault classification (Figure 4), used to compose

Figure 3, includes four time subdivisions:

(1) Historic - to approximately 200 years before

present;

(2) Holocene - to approximately 11,000 years before

present;

(3) Late Quaternary - to approximately 500,000

years before present;

(4) Quaternary - to approximately 2-3,00,000 years

before present.

Subdivisions (1) and (2) represent active faults while

subdivisions (3) and (4) represent potentially active

faults (Figure 3).

The important Point Mugu earthquake of February

21, 1973 and its aftershock sequence have not been analyzed

10

Figure 4. Geologic time scale used to classifyfault activity, adopted from Ziony and others, 1974. After Byer (1975).

11

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earthquake catalog for the study area from 1973-1976.

The Study Area: Major Faults

The study area is delineated by deep-seated, steeply-

dipping, and seismically-active faults. Figure 3 shows

the portions of these fault traces which have moved during

Quaternary time, based on geologic and geomorphic evi­

dence, discussed later.

The Los Angeles physiographic basin is 80 kilometers

long and 32 kilometers wide, located along the coast and

bounded to the north by the Santa Monica fault system

(Figure 3) and to the northeast and east by the Whittier

fault zone (Figure 1). The basin's southern and south­

eastern boundaries are marked by the San Joaquin Hills

and the Santa Ana Mountains.

The Santa Monica fault zone is an active north-

dipping high-angle reverse fault across which the Santa

Monica Mountain structural block has been thrust south­

ward over the alluvial fill of the Los Angeles basin since

Pliocene time (Yerkes and others, 1965). This fault

continues to the east as the Raymond Hill fault and to

the west as the Malibu Coast fault and the offshore Dume

fault (Figure 3). Geologic evidence for latest movements

13

across these fault extensions is north-over-south high-

angle reverse displacement.

The only disruption along the east-northeast trend­

ing Santa Monica fault zone exists in the Elysian Park-

Puente Hills area where sgemented faults comprising the

northwestward extension of the Whittier fault zone break

the continuity between the Santa Monica fault and the

Raymond Hill fault (Figure 5). Since the Los Angeles River

flood plain and alluvial deposits have obscured the fault

traces at the surface, the crosscutting relationship be­

tween these two fault systems is presently subject to

debate (Lamar, 1970; 1975).

The north-northwest-trending Whittier fault zone

defines the eastern boundary of the Los Angeles basin and

is most significantly represented in this study area by

its northwesterly extension which intersects the Santa

Monica fault zone. It is a continuous fault trace along

the southwestern front of the Puente Hills where it has

offset Miocene strata up to 4250 meters along a reverse

fault dipping 65° to 75° NE (Yerkes and others, 1965).

The Newport-Inglewood fault zone interrupts the

otherwise slight seaward grade of the Los Angeles basin

and is expressed as a series of low hills extending from

Beverly Hills southeast to Huntington Beach. The low

hills and mesas within the otherwise expressionless

alluviated lowlands are a series of en echelon corapres-

14

Figure 5. Geologic map detailing proposed connec­ tions between the Hollywood fault (Santa Monica fault) and the Raymond Hill fault, Generalized from Woodford and others(1954) and Lamar (1970) . Af^ter Lamar(1975).

Q: alluvium; Tf: Fernando Formation Tp: Puente Formation; Tt: Topanga Formation; TV: Volcanic rock; Kp: Plu­ tonic + metamorphic rock.

trace 2: Ziony and others (1974) trace 3: Woodford and others (1954)

15

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sional anticlines which are the result of a deep-seated

right-lateral wrench fault process (Wilcox and others,

1973; Harding, 1973).

The Palos Verdes Hills fault (Figure 3) subparallels

the Newport-Inglewood fault zone, is seismically active,

and displays a deformational mechanism of high-angle

reverse faulting. On the Palos Verdes peninsula and to

the south, in the San Pedro Bay, the fault dips steeply

to the southwest (Woodring and others, 1946; Fischer and

others, 1977). The northwesterly trace of the Palos

Verdes Hills fault within the Santa Monica Bay, inter­

preted from subbottom acoustic seismic profiles, terminates

against the east-trending Dume fault (Figure 3), where only

insignificant amounts of strike-slip displacment are

apparent (Greene and others, 1976; Junger and Wagner, 1977;

Nardin and Henyey, 1978). Vertical separation appears to

terminate at the top of the lower Pliocene strata (Junger,

1976; Nardin and Henyey, 1978).

Southern California Seismic Network

In 46 years, since its inception in 1932, the Southern

California Seismic Network has grown to approximately

120 short-period and long-period seismometers. Offshore

seismic coverage has been aided by the placement of

seismometers on the Channel Islands. Figure 6 shows the

17

Figure 6. Seismic stations monitored and maintained by the CIT and USGS, as of January 1, 1977. The USGS maintains the stations outlined in the southeastern corner of California.

18

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spatial distribution of seismic stations presently moni­

tored by the seismological laboratories at the California

Institute of Technology (CIT) and the United States

Geological Survey (USGS).

This regional coverage has been augmented, beginning

in 1971, by specialty networks set up in the Los Angeles

basin by the University of Southern California (USC).

In addition, a temporary network to locate and study the

aftershocks of the 1973 Point Mugu earthquake was jointly

deployed by the USGS and CIT at the western edge of this

study area during the months of February, March, and April,

following the main shock of February 21, 1973.

Since 1972, USC has established two local seismic

networks along the Newport-Inglewood fault zone in the

western Los Angeles basin. Stations within the Baldwin

Hills Seismic Network (BHSN) and the Long Beach Seismic

Network (LBSN) are located in Figure 7 and are listed

in Table 1. Both networks serve to study the micro-

earthquake activity associated with the tectonically

active Newport-Inglewood fault zone, which is currently

subject to substantial oil removal and fluid injection

programs. Also shown in Figure 7 and Table 1 are stations

located along the periphery of the Los Angeles basin,

telemetered to both USC and CIT. This recording overlap

provides a cross check on the timing and location of

events occurring within the Los Angeles basin. Common

20

Figure 7. Location map for the Baldwin Hills Seismic Network and the Long Beach Seismic Network. See Table 1 for station coordinates. After Teng and Henyey (1973).

21

to

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TABLE 1.

Seismic Network in the Los Angeles basin area.

SEISMIC NETWORK IN THE LOS ANGELES BASIN AREA

Station Code

BER

HCM

IPC

TPR

J3F

GPP

LCM

DHS

DHT

DRP

LCL

FMA

LNA

RCPSCRa

SJRa

VPDa

TCNa

CISa

MWC

PAS

Latitude, N

34°00.51'

33°59.64 T

33°58.24'34°05.33'

336 59.58'34°07.76'

34°01.07 t34°01.05'

33°45.06'

33°46.72'

33°50.00'

33°42.75'

33°47.35'

33°46.66'

34°06.37'

33°37.20'

33°48.90'

33°59.67 f

33°24.40'

34°13.40'

34°08.90'

Longitude W

118°21.72 I

118°22.98'

. 118°20.07 I118°35.20 T

118°20.68'118°18.59'

118°17.22'118023.13'

118°13.25 r

118°12.27 f

118°11.55'

118°17.12 I

118003.27'

118°08.00'

118°27.25'

117°50.70'

117045.70'

118000.77'

118°24.40'118°03.05 T -

118°10.30'

Remarks

Began July, 1975

Term. July, 1975

Began Apr. , 1975

Tiiree Components

After Teng and others (1975).

Superscript a designates stations monitored at both USC and CIT.

23

seismic records from these overlapping networks permit

identification and elimination of nearly all errors

involved in the reading, measuring, and recording of

daily local seismic events from 1973 to 1976. The cross­

over of seismic data between the networks in the Los

Angeles basin permits a double check on station polari­

ties, as they are determined from nearby controlled quarry

blasts originating in the Mojave Desert.

The BHSN and the LBSN are small-aperture seismic-

station clusters employing short-period vertical seis­

mometers (T = 1.0 sec), specifically designed to monitor

local microearthquake activity. By combining these local

clusters with the peripheral Los Angeles basin seismic

stations and the regional CIT-USGS seismic stations, out

to a 120 kilometer radius from the Los Angeles basin,

every event in the study area with a magnitude, M a.2.0

has been recorded and relocated with improved precision.

Appendix A includes all the stations used in this

study. Station installation dates have been variable

and many stations have become operable only recently.

Enough changes have occurred locally and regionally since

1973 to warrant revising the master list of stations for

the 1976 earthquake data set. Table 6 (Appendix A)

is a list of stations from which data has been taken to

relocate the events occurring from 1973 through 1975;

24

Table 7 (Appendix A) is a revised master list of stations

used to relocate only 1976 events.

Crustal Velocity Model

Since the development of a specific crustal velocity

model for the study area is beyond the scope of this

project, Healy's (1963) model has been chosen to appro­

ximate the crustal velocity structure for the general

western Los Angeles basin and the Santa Monica Mountain

region (Table 2).

Table 2

Crustal Velocity Structure

Layer Depth (Km) P Velocity (Km/sec)

1 0.0 to 2.6 3.0

2 2.6 to 16.7 6.1

3 17.6 to 26.1 7.0

4 below 26.1 8.2

S-wave velocity is assumed to be 0.56 times the P-wave

velocity.

Six studies have proposed different crustal velocity

structures for southern California. Four models are

applicable to travel paths within this study area; the

other two velocity structures have been derived for

25

the Mojave Desert region to the east. Figure 8 shows

the basis for the six crustal velocity models for southern

California.

The models are compared in Figure 9. All consider

the crust to be composed of three layers overlying a

uniform half-space (upper mantle). All distinguish a

low-velocity surface layer which varies from 3.0 km/sec

along the southern California coast to 5.5 km/sec within

the Transverse Ranges and Mojave Desert. Each study has

established a distinct upper crustal layer, with a

velocity of approximately 6.2 km/sec and an intermediate

crustal-velocity layer of 6.8-7.0 km/sec, with a low

of 6.2 km/sec in the Los Angeles basin (Teng and Henyey;

1973) and a high of 7.66 in the Mojave Desert (Press;

1960). The Pn velocity is typically 8.1 to 8.2 km/sec,

at a depth of approximately 30 to 35 km. Hadley and

Kanamori (1977) have discovered an anomalously high velo­

city of 8.3 km/sec for the upper mantle, underlying the

Transverse Ranges.

The Los Angeles basin crustal velocity model pro­

posed by Teng and Henyey (1973; Figure 9) is a deriviative

of Healy's model for a portion of the Los Angeles basin

and thus should not necessarily be extrapolated to a

more regional seismic station data base.

Stierman's and Ellsworth's (1976) velocity model,

derived from artificial explosion travel-time data for

26

Figure 8. Plan view of areal extent for eachcrustal velocity survey conducted in southern California. Shaded area represents study area. Base map after Hileman and others (1973).

H-H 1 = Healy (1963); R-H = Roller and Healy (1963)

T-T 1 = Transverse Ranges (Hadley and Kanamori, 1977)

K-K = Mojave Desert (Kanamori and Hadley, 1975)

P-P = Mojave Desert (Press, 1960)

M = Point Mugu (Stierman and Ellsworth, 1976)

L = Los Angeles Basin (Teng and Henyey, 1973)

27

38°-

Lake Mead

112

28

Figure 9. Cross section of the six crustal velo­ city models proposed for various portions of southern California. Depths and interval thicknesses are in kilometers.

29

ST

IER

MA

N

a

TE

NG

ft

HA

DLE

Y

a

KA

NA

MO

RI

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ELL

SW

OR

TH

(1

976)

ME

ALY

/ U

SG

S (

1963)

HE

NY

EY

(

19

73

) K

AN

AM

OR

I (1

977)

HA

OLE

Y (

1975)

PR

ES

S(I

96

0

Pt.

Mug

u S

anta

M

onic

a LA

B

asi

n

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jave

M

oja

veB

ay

(Wes

tern

Ext

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1.5

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28.8

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6.8

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4.2

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27.0

JL .

1A

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34

.0

42

.0

5K

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6.7

7.8

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1 ~

^--^^ -*

v.

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41

.8

3.0

6.11

7.6

6

8.11

the 1973 Point Mugu earthquake study, should also be

discounted as a regional crustal structure due to its

localized nature. However, their travel-time plot for

the upper crustal region along the coastline supports

Healy's model (Figure 9).

Kanamori and Hadley's (1975) Mojave Desert region

crustal velocity model is presently used to relocate

events within the CIT seismic network. This model

assigns a 7.8 km/sec value to the upper mantle. This

low value is typical of the Basin and Range structure to

the east (Herrin and Taggart, 1962; Cleary and Hales,

1966; Archambeau and others 1969). The existence of such

a low Pn velocity for coastal California has not been

substantiated elsewhere in the literature. Healy's

value of 8.2 km/sec is in closer accordance with Press*

(1960) value of 8.1 km/sec and Hadley and Kanamori*s

(1977) Pn velocity of 8.3 km/sec for the Transverse

Ranges. Since both Mojave models (Press, 1960; Kanamori

and Hadley, 1975) do not include data within this study

area, neither can be as dependable as local models for the

Los Angeles basin.

Hadley and Kanamori's (1977) crustal velocity model

for the Transverse Ranges describes a locally anomalous

velocity pehnomenon occurring strictly beneath this

east-west trending structure. Its regional applicability

31

is doubtful and also should be discounted in terms of a

local model.

For a crustal model to be useful it must remain

sensitive to the velocity structure within the earthquake

focal region while closely approximating travel times at

distant seismic stations. Healy's model meets these

criteria. Independent crustal velocity studies in coastal

southern California, by Teng and Henyey (1973) and Stier-

man and Ellsworth (1976), have corroborated Healy's choice

of surface layer (3.0 km/sec) and upper crustal (6.1 km/

sec) velocities as representative of the study area.

Healy assigns velocities of 7.0 km/sec to the intermediate

crustal layer and 8.2 km/sec to the upper mantle. In

light of all six models proposed for southern California

(Figure 8), Healy's velocities represent regional averages.

Station Delays

Healy's model has been generalized herein to represent

the local velocity structure for the study area as well

as the regional velocity structure for southern California.

In addition, station delays have been computed and assigned

to individual seismic stations. These station delays

adjust the arrival time at each seismic station to account

for travel paths which are affected by azimuth-dependent

and localized velocity variations. The assignments of

32

individual station delays have been computed using a sub­

routine from Lee and Lahr's (1975) computer program,

HYPO71(REVISED), as outlined below.

Each station correction constant accounts for velo­

city variations due to: (1) the juxtaposition of differ­

ing geologic structures and lithologies resulting from

Cenozoic tectonism and recent fault movements, such as

the contrasting crystalline basement complexes juxta­

posed northeast and southwest of the Newport-Inglewood

fault zone in the Los Angeles basin (Woodford, 1924;

Bailey and others, 1964; Yerkes and others, 1965); (2) tra­

vel-time delays, ranging from 0.3 to 0.7 seconds (Kana-

mori and Hadley, 1975), for seismic stations in the Los

Angeles and Ventura basins, where the sediment pile is

estimated to reach 9150 meters (Yerkes and others,

1965; Vedder and others, 1974), (3) P-wave travel-time

advances, ranging from 0.1 to 0.3 seconds, as a result

of travel paths partly within an anomalously high-velocity

upper mantle of 8.3 km/sec which exists beneath the

Transverse Ranges (Hadley and Kanamori, 1977); and

(4) a minor, yet necessary, elevation correction.

Hadley and Kanamori (1977) assume a range of near-surface

velocities of 3.0 to 5.0 km/sec to calculate elevation

corrections for individual stations within the CIT

seismic network. Assuming a maximum travel-time of 3.0

km/sec, the maximum effect of elevation variations between

33

Table 3. Explanation of earthquake relocationquality rating and data output summary lists found in Appendices C and D. After Lee and Lahr (1975).

34

Approximate accuracySolution Quality

A (excellent)B (good)C(fair)D (poor)

Epicenter (km)

12.55

>5

Focal Depth (km)

25

>5>5

The following data are given for each event:(1) Origin time in Greenwich Civil Time: date, hour (HR), minute (MN), and second (SEC). If **

precedes the date, it indicates that this event also appeared in the Caltech catalog.(2) Epicenter in degrees and minutes of north latitude (LAT N) and west longitude (LONG W).(3) DEPTH, depth of focus in kilometers.(4) MAG, Richter magnitude of the earthquake.(5) NO, number of stations used in locating earthquake.(6) GAP, largest azimuthal separation in degrees between stations.(7) DM1N, epicentral distance in kilometers to the nearest station.(8) RMS, root-mean-square error of the time residuals: RMS = [£i(/?< 2/M>)]* where Rt is the

observed seismic-wave arrival time minus the computed time at the ith station.(9) ERH, standard error of the epicenter in kilometers: ERH = [SDX2 -SDY3]*. SDX and SDY

are the standard errors in latitude and longitude, respectively, of the epicenter. When NO ^ 4, ERH cannot be computed and is left blank.

(10) ERZ, standard error of the focal depth in kilometers. When NO ^ 4, ERZ cannot be computed and is left blank. It ERZ > 20 km, it is also left blank.

(11) Q, solution quality of the hypocenter. This measure is intended to indicate the general reliability of each solution.

Q Epicenter Focal Depth A excellent good B good fair C fair poor D poor poor

Q is based on both the nature of the station distribution with respect to the earthquake and the statistical measure of the solution. These two factors are each rated independently according to the following schemes:

Station DistributionQ NO GAP DMIN A £ 6 ^ 90e £ Depth or 5 km B £6 < 135s < 2 x Depth or 10 km C £6 £180° £ 50 km

Statistical MeasuresQ RMS (sec) ERH (km) ERZ (km) A < 0.15 < 1.0 < 2.0 B < 0.30 < 2.5 < 5.0 C < 0.50 < 5.0 D Others

Q is taken as the average of the ratings from the two schemes, i.e., an A and a C yield a B. and two B's yield a B. When the two ratings are only one level apart the lower one is used, i.e., an A and a B vield a B.

35

stations can only account for 0.3 seconds in travel time.

The station elevation correction, upper-mantle P-wave

advance, basin-alluvial P-wave delay, and lateral velocity

variations across faults and structural blocks are statis­

tically combined and smoothed by the calculated station

delays, listed in Appendix A.

During the 1970's, southern California's seismic

network has increased to over 120 recording sites. This

comprehensive coverage, lacking only in the offshore

region, affords a statistical advantage for improving

the relocation of earthquakes. The scheme utilized here

is a least-squares reduction of the travel-time residuals

using only good to excellent quality earthquake reloca­

tions. For every earthquake, the P and S arrivals from

each station list are first routinely edited and weighted,

as described in the procedure, using no station delays.

From the statistical measures and station distribution,

determined from the HYPO7KREVISED) output, each earthquake

is quality rated for its hypocentral relocation and placed

in a category, "A" through "D," as explained in Table 3.

All "A" and "B" quality relocations have an epicentral

uncertainty of +2.5 kilometers and a hypocentral uncer­

tainty of +5 kilometers. Only "A" and "B" quality loca­

tions have then been fitted to the least-squares routine.

The least-squares process places each earthquake at the

appropriate hypocenter which minimizes the difference

36

(root-mean-square for all stations) between the

actually observed seismic-wave travel time at each

station and the travel time calculated for the same travel

path using Healy's layered crustal-velocity model. The

assigned station delay becomes the average root-mean-

square (Table 3) of the residual time difference between

the observed and calculated travel times. Appendix B

displays the improved quality of hypocentral locations

resulting from three iterations of this least squares

routine.

37

PROCEDURE

Data Collection

For every earthquake located within the study area,

data from individual seismic stations have been gathered,

collated, and merged from all seismic networks in southern

California. Station data consist of the P-wave arrival

time (+ .02 sec), its first motion, the S-wave arrival

time with its sense of first motion, and the duration of

the event (+ 5 sec). This information from each station

has been read directly from records from which a computer-

card phase list has been generated.

Initially, USC, CIT, and the USGS had each created

a set of preliminary earthquake relocations by only in­

corporating the station data from their respective seismic

networks. From these preliminary earthquake data sets,

all earthquakes located within or at the periphery of the

study area were considered for subsequent refinement in

relocation. These three data sets (USC, CIT, and USCS)

were next collated, merged, and then processed by Lee

38

and Lahr's (1975) computer program, HYPO7KREVISED) ,

to determine the hypocenter, magnitude, and first-motion

pattern for these local earthquakes.

Often, seismic records from stations had to be

reread to complete the data catalogue for each earthquake.

For events through 1975, station data from the USGS

Santa Barbara Channel Network had only been catalogued

for locations west of longitude 118° 30'W (approximately

offshore of Santa Monica). Consequently, station records

had to be read for all possible events occurring in the

Los Angeles basin, lying to the east of 118 30'W.

Under the direction of W.H.K. Lee of the USGS

National Center for Earthquake Research, I initially

assisted in relocating 1972-1975 events for the area

bounded by 33° 45'N to 34° 45'N latitude and 118° 30'W

to 120 30'W longitude. The eastern boundary of the

above data set overlaps into this study area as outlined

in Figure 10. These earthquake relocations are based on

station data from the USGS Santa Barbara Channel Network

supplemented by data supplied by peripheral USC and CIT

seismic stations. Therefore, the 1973-1975 earthquake

relocations for the westernmost portions of this study

area to longitude 118° 30'W are discussed by Lee and

others (1979) and are only presented here. This data

subset includes the Point Mugu earthquake of February 21,

1973. East of Point Dume, 1973-1976 earthquake relocations

39

Figure 10. Shaded region represents overlap of 1973-1975 station data between this study and USGS study by Lee and others (1979). Approximate locations for eight-station network installed in the Santa Barbara Channel by USC in August, 1978.

Sm Is. = San Miguel Island

SR Is. = Santa Rosa Island

SC Is. = Santa Cruz Island

A Is. - Anacapa Island

40

N

Sa

nta

B

arb

ara

are more dependent on the USC and CIT networks, while the

USGS Santa Barbara Channel Network assumes a peripheral

importance. CIT has assumed operation of the USGS Santa

Barbara Channel Network since the beginning of 1976.

Consequently, all earthquakes occurring in 1976 near the

western boundary of the study area (119 O 15'W) are relocated

and discussed here.

Data Processing

Seismic station data obtained in this study have been

processed by HYPO71 (REVISED), a computer program by Lee

and Lahr (1975) which produces an output for each earth­

quake consisting of its origin time, hypocenter location,

magnitude, and plot of the first-motion pattern. Addi­

tional statistics, included in the output, described the

precision of each earthquake relocation. HYPO71

(REVISED) has been adapted from an earlier version for

worldwide data (Lee and Lahr, 1972).

Three forms of input data are required by HYPO71

(REVISED). These are: (1) the geographic coordinates

for each seismic station (+0.1 km, whenever possible);

(2) a layered, crustal velocity model, preferably con­

trolled by explosion experiments; and (3) precise P-wave

(+ .02 sec) and S-wave (+ .05 sec) arrival times.

42

Data Editing and P Residuals

In order to reach its final refined form, the data

output from HYPO71 (REVISED) has been subjected to editing

and computer processing for five iterations, as outlined

by the flow diagram in Figure 11. This editing process,

in almost every case, has improved the accuracy of the

earthquake epicenters. The computer output for three

typical earthquakes, in final form, is shown in Appendix

C. An abbreviated explanation of the output, pertinent to

this discussion, is included in the appendix.

For every earthquake, each phase list card has been

considered individually and edited with respect to four

statistical variables in the computer output: (1) the

root-mean-square P-wave residual (RMS); (2) the number

of seismograph stations recording the event (NO); (3)

the maximum azimuthal gap between the recording stations

(GAP); and (4) the minimum distance between the trial

epicenter and the closest recording station (DM). These

variables, as well as the other parameters presented in

each output iteration of HYPO71 (REVISED) are explained

in Table 3.

Earthquake hypocenters are determined according to

Geiger's (1912) method, which minimizes the arrival-time

residuals, in a least-squares sense, between the observed

43

Figure 11. Flow diagram outlining editing pro­ cedure.

44

DATA COLLECTION (for each earthquake)

USGSSeismicNetwork

I

rereadindividualseismograms

Data read from seismograms/recorded on IBM cards

P-wave arrival time P-wave 1st motion Duration

S-wave arrival time S-wave 1st motion

DATA PROCESSING (for each earthquake) v________

HYP071 (Revised) computer program (methods by Geiger (1912) Eaton (1969)

Seismic station coordinates

OUTPUT RMS = RMS _,, n n+1

Individual

NO, DM,

IF RMS <- n

Station

GAP, P-RES

- 0 .10 or RMS - RMS ^ 0.05 n n-1

Statistics

Tt

epicenter location focal depth magnitude duration

DATA EDITING & P-RESIDUALS

__I ____IF DM>120 to 150 Km N0±8, THEN WT=3

N0>8, THEN WT=4

IF GAP>135 or NOj-8, THEN add reliable S-wave data

IF GAP^-135 and- THEN no need to add S-wave data

IF GAP>90 , THEN, only downgrade to a maximum of 3 (not 4)

IF P-RES>1.00 andc , THEN WT=3

N0>8, THEN WT=4

IF 0.40^P-RES-«-1.00 THEN, downgrade accordingly

45

station arrivals and the corresponding calculated station

arrivals. Since Geiger's (1912) method, when used for

hypocenter relocation, is a summation of least-squares

residuals exising at all of the involved seismograph

stations, the editing process is actually a weighting

process through which the editor can eliminate a station

or downgrade a station's influence on the hypocenter

relocation. In this manner, editing can eliminate or

downgrade the phase list card for any station which may

have an incongruent or inconsistent P-arrival reading and

which may, in turn, have a strong influence on the least-

squares relocation of the event. The weighting options

are as follows:

Weighting Symbol Explanation

0 or "blank" full weight1 3/4 weight2 1/2 weight3 1/4 weight4 no weight

The several closest recording stations to an event

are usually not downgraded since these stations are criti­

cal in determining the focal depth of the earthquake.

Only when the distance between the epicenter and the

closest recording station is less than the actual focal

depth (DM<DEPTH) can the hypocenter be considered to be

reliably located. On the other hand, the computer pro­

gram slightly downgrades the contribution of each station

46

with increased distance from the trial hypocenter. It

is general practice to exclude stations from the phase

list which lie more than 120-150 kilometers away from

the trial epicenter.

If the number (NO) of seismic stations recording the :I

event is initially small (<8) , then the phase list cards

are usually only downgraded to a minimum of "3" (k weight) ;

rather than to "4" (no weight), since elimination of any

stations in this case (N0<8) usually increases the '

azimuthal gap (GAP) between the remaining recording sta- :

tions. Ideally, the value of GAP should be less than 90°

to properly constrain the hypocenter relocation. If the

GAP<135°, the S-wave arrival data from the phase list, if

reliable, is considered in the relocation process. If

GAP<135° and N0<15, the S-wave arrivals usually will

not significantly improve the earthquake relocation.

The root-mean square summation of the P-wave residuals

(P-RES) for each phase list card results in the RMS

value for each earthquake. In general, the P-RES values

for each event have been appropriately weighted such that

the resultant RMS value is less than .25 sec (RMS<.25 sec).

If P-RES<1.0 sec, the phase list card is excluded from

further consideration unless it is critical to the earth­

quake relocation. In this case, the original seismic

record has been reread to determine the exact P-wave

arrival time. For the situation 0.4<sec P-RES<1.0 sec,

47

the phase list card has been appropriately downgraded

or eliminated. The final weighting of each phase list

card is a combination of the editor's weighting and the

computer's distance weighting formula. The resultant of

this combination is listed under the heading P-WT, as

shown in the three earthquakes in Appendix C, for each

phase list card.

48

RESULTS

Four hundred twenty three earthquakes, occurring

between January 1973 and December 1976 in the study area

(Figure 12), have been relocated and are summarized in

Appendix D. This summary of information for each earth­

quake includes origin time, epicentral location, focal

depth, duration magnitude, number of recording stations,

azimuthal gap, distance from epicenter to closest record­

ing station, root-mean-square travel-time residual, epi­

central location error, focal depth error, and a statisti­

cal quality rating for each earthquake relocation. The

complete output summary for each earthquake includes the

phase-card list, the iterative step-wise locational

procedure used by HYPO71 (REVISED), the statistical

information describing the earthquake relocation, and the

first-motion plot of the P-wave readings from the phase

list. A sampling of the HYPO71 (REVISED) output for three

typical earthquakes is provided in Appendix C the complete

listing is on file at USC.

49

Figure 12. Epicenter plot for all 423 earthquakes relocating in the study area during 1973-1976.

50

All 423 earthquake epicentral locations are plotted

in Figure 12. Also plotted for the 423 events are

magnitudes (Figure 13) and focal depths (Figure 14),

which correspond to the epicentral locations plotted in

Figure 12. Magnitudes are calculated on the basis of

the method described by Lee and others (1972) in which

the magnitude of an event is considered to be the average of

the individual station magnitudes, as determined from

their signal durations. The signal duration, tau (T)

is based on the USGS develocorder records. The termination

of the signal is defined by an amplitude of less than one

centimeter on the develocorder output. The magnitude of

an event at a recording station is given by Lee and

others (1972):

m = -0.87 + 2.00 logT + 0.00035A

where

M = magnitude

T = signal duration in seconds

A = epicentral distance in kilometers

Focal depths are plotted in Figure 13 for only "A" and

"B" quality relocations. "C" and "D" earthquakes have

no defined range of error in focal depth but have been

assigned arbitrary focal depths by the computer.

The map size (scale 1:250,000) has been chosen to

conform to the size and scale of other published maps

52

Figure 13. Magnitude plot of 1973-1976 earth­ quakes

-f = Magnitude > 1.0

& = Magnitude > 2.0

O = Magnitude > 3.0

Q = Magnitude > 4.0

^ = Magnitude > 5.0

53

in

Figure 14. Focal depth plot for 1973-1976 "A"and "B" quality earthquake relocations only.

-f = Depth < 4.0 Km

& = Depth < 8.0 Km

^ = Depth = 8.0 Km

O = Depth < 12.0 Km

D = Depth > 12.0 Km

55

56

(e.g., the California Division of Mines and Geology state

geologic maps). They are included in Appendix E for

reference. Each of these maps has been reproduced at

a smaller scale in the text.

The margin of error for "A" quality (Table 3) epi-

central locations is + 1.0 kilometer; for "B" quality

locations, +2.5 kilometers. On a map scale of 1:250,000

(Appendix E) epicenters are plotted as open circles which

represent a 3/4 kilometer radius. Therefore, errors in

relocation exceed the boundaries of the circles for all

events. Fault traces are plotted to within approximately

300 meters (Ziony and others, 1974). The most dependable

and accurate relocations, namely "A" and "B" quality

events, are plotted in Figure 15. The regional geogra­

phical bias of seismic station locations tends to group

the more accurate relocations onland rather than off­

shore.

As shown in Figure 12 (also Appendix E), the 1973

Point Mugu earthquake and its aftershocks cannot be indi­

vidually deciphered on a plot with a scale of 1:250,000.

In order to display these data more meaningfully, the

region bounded by 34° Ol'N and 34° 07'N latitude and

118 56'W and 119 04'W longitude has been expanded to a

scale of 1:83,333. The same plots as above have been

produced for this cluster of events and are presented in

Appendix F.

57

Figure 15. "A" and "B" quality earthquake reloca­ tions, 1973-1976.

= "A" quality relocation

= "B" quality relocation

58

m

For every earthquake relocation, the HYP071 (REVISED)

output produces a P-wave arrival first-motion plot, which

is a lower-hemisphere projection onto the Wulff-net

equatorial plane. From the 423 earthquakes, 26 individual

solutions were sufficient by themselves to produce enough

station control to define a fault plane (Figure 16 and

Table 4). In addition, 11 composite fault-plane solutions

have been created by combining groups of earthquakes

related either in time or space (Figure 17 and Table 5).

The choice of fault planes is discussed later in terms

of the local geology, structure and seismicity patterns.

The composite fault-plane solutions have been drawn

under the assumption that all the earthquakes define

the same fault or fault zone and are produced by the

same focal mechanism. Due to this assumption, composite

fault-plane solutions are more susceptible to error than

individual fault-plane solutions. The advantage of the

composite fault-plane solution is to incorporate data

points from local events into the data set of a well-

located earthquake to further constrain the choice of

possible fault planes. Combining data from several

small events was often sufficient to formulate a consistent

first-motion pattern, defining a fault-plane solution.

The focal mechanism for each earthquake is assumed

to be a double-couple dislocational model (Stauder, 1962).

Thus, the plane of fault movement becomes one of two

60

Figure 16. Fault-plane solution map for individual earthquakes, 1973-1976. Black quadrants indicate compression; white quadrants indicate dilitation.

61

TABLE 4.

List of individual fault-plane solutions

Solution Event YrMonDy HrMin Sec Lat N Long W

1234567891011121314151617181920212223242526

179181167255191301124406368367333410084280396205202265334269199257365210358282

731129731209731118741022740117750504730520761115760715760627751007761122730308750123760929740312740306741206751011741219740227741106750620740331760504750128

11251536072912130830201618081208022722112333175523490348070007350152134506551236054100380051112000550522

18.8713.6959.7338.6951.. 0533.4242.4703.7739.5436.1726.6911.0551.8142.4251.0945.2634.0113.0800.5916.5817.9327.5528.9146.7629.7122.85

34-03.7334-00.3333-58.3433-58.5733-51.9933-57.1733-56.5133-56.4933-51.9434-03.0133-52.5533-56.2433-49.5833-54.0934-06.0034-06.1834-06.8934-07.1034-06.7234-04.5734-04.7734-13.1633-59.8834-01.0634-06.2234-11.77

119-02.48118-02.48118-25.26118-22.57118-24.97118-19.34118-25.91118-20.75118-20.24118-08.38118-29.03118-37.54118-40.57118-42.08118-18.18118-13.75118-15.34118-15.83118-04.86118-06.27118-05.99118-10.40118-49.07118-44.63119-01.53118-39.07

63

Figure 17. Composite fault-plane solution map forearthquakes, 1973-1976. Black quadrants indicate compression; white quadrants indicate dilitation.

64

<y\

TABLE 5.

List of composite fault-plane solutions

Solution Event YrMonDy HrMin Sec Lat N Long W

A 179181302397

B 167168

C 322347349378399

D 117150348403

E 124293336338383406

F 368369

G 101291327

H 81119318373491

I 81373379401415

731129731209750504761005

731118731118

750804760204760207760818761013

730424740826760206761019

730520750327751027751125760902751115

76-715760716

730329750307750927

730306730511750722760727761018

730306760727760822761018761129

1125153620272224

07290733

07161843201817180112

1454202818441504

180806520207023602561208

02270327

213516291247

06270235061809000551

06270900095305510040

18.9713.6916.7115.48

59.7347.62

15.2056.2612.4857.6025.87

57.8714.1556.3316.90

42.4749.4015.9257.3546.9403.77

39.5439.61

33.9633.8033.72

37.4036.2517.7650.9215.25

37.4050.9247.2415.2501.81

34-00.2734-00.3333-58.7234-01.15

33-58.3433-57.88

34-01.3633-59.8033-59.0633-00.4733-59.82

34-00.2434-01.1234-02.2933-60.00

33-56.5133-58.3433-58.2133-56.9133-55.7933-56.49

33-51.9433-51.68

34-00.6734-00.5633-59.76

33-55.4633-58.9433-57.6733-55.0333-56.32

33-55.4633-55.0333-54.9833-56.3233-56.05

118-25.54118-25.48118-23.82118-25.37

118-22.26118-23.23

118-23.61118-21.18118-20.36118-22.19118-21.09

118-19.80118-20.54118-21.51118-20.38

118-20.91118-20.77118-21.09118-20.56118-18.84118-20.75

118-08.24118-09.24

118-32.23118-30.95118-31.63

118-32.02118-32.76118-32.24118-31.61118-31.96

118-32.02118-31.61118-32.87118-31.96118-30.08

66

Table 5 (continued)

J 409 761122 1753 10.96 33-58.59 118-38.42410 761122 1755 11.05 33-56.24 118-37.54411 761122 1806 12.94 33-56.95 118-33.17412 761122 1932 36.49 33.57.44 U8-37.98

K 407 761121 0009 46.17 33-59.79 119-09.98408 761121 1939 44.91 34-01.11 119-10.11

67

orthogonal nodal planes. This orthogonality criterion,

implying equal amounts of dislocation on both sides of a

fault, has been shown to deviate from 90° by a maximum

of only 10° to 20° (Sykes, 1967). The direction of the

inferred axes of maximum compression and tension lie

at 45 angles to the nodal planes and in a plane perpen­

dicular to the two nodal planes. The axis of maximum

compression bisects the tensional quadrants.

The P-wave first-motion data which have produced

reliable fault-plane solutions and accompanying suggested

focal mechanisms for each of these events are presented

in Figures 20 to 32, in the "Discussion." Appendix G

addresses possible problems capable of limiting the

reliability of fault-plane solutions. One of these pro­

blems, the variance of a fault-plane solution as a function

of the crustal velocity model, is investigated for all

six possible models presently suggested for southern Cali­

fornia .

A local stress pattern, derived from fault-plane

solution data, is presented in Figures 18 and 19. The

compressional stress direction for each fault-plane solu­

tion has been projected onto the horizontal plane; thus,

only a component of the true stress direction is shown.

The maximum compressive stress is assumed to correspond

with the P axis of the fault-plane solution. Azimuthal

errors in stress direction determinations have been

68

Figure 18. Compressive stress pattern for the study area derived from 1973-1976 individual fault-plane solutions. Dots represent epicenters for fault-plane solutions. Lines directed away from each epicenter represent the horizontal projection of the compressional stress vector, derived from the fault-plane solution. The magnitude of the stress is not indicated by the length of the line.

69

SA

NT

A

MO

NIC

A

MO

UN

TA

INS

4° 0

0' 0

__

__

9

0B

\

Figure 19. Compressive stress pattern for the study area derived from 1973-1976 composite fault-plane solutions.

71

OX

NA

MO PL

AIN

AN

TA

M

ON

ICA

M

OU

NT

AIN

*

4

*00

Km Mll

«t

11 0

0M

t*B

O'

PA

LM

V

IRD

t*

MIL

L*

to

reviewed by Sbar and Sykes (1978). This error can

reach + 40 for strike-slip faulting while it is smaller

(<20 ) for normal- or reverse-slip faulting (Raleigh,

1972; Sbar and Sykes, 1978) . Even though a large margin

of error is inherent in the stress directions derived from

strike-slip solutions, the large number of measurements

(37) and the dominance of reverse-fault mechanisms within

this region statistically precludes such a large error

from affecting the regional trend of the data. Figure

18 shows variations in derived compressive stress direc­

tions of up to 90 . These opposed stress directions

appear to be caused by local fault geometries, as discussed

below.

73

LATEST FAULTING: PREVIOUS WORK

Newport-Inglewood Fault Zone

Recent tectonic activity on the Newport-Inglewood

fault zone (Figure 3) is marked by a series of en echelon

anticlines which have formed in Quaternary basin allu­

vium. The 1933 Long Beach earthquake, M = 6.3 (Richter,

1958), with an epicenter located offshore of Newport

Beach, California, attests to the fault's historic active

status. Although no surface faulting has been recognized

during historic time, subsurface displacement during the

1920 Inglewood earthquake, M = 4.9 (Tabor, 1920), and

during subsequent shocks in the early 1940's, have damaged

oil wells in the Dominguez and Rosecrans oil fields

(Bravinder, 1942). The sense of shearing, from strati-

graphic correlations in oil wells, is right-lateral.

Latest faulting locally cuts upper Pleistocene

terrace deposits (Ziony and others, 1974). A similar

age for latest faulting also applies to the offshore,

southwest extension of the Newport-Inglewood fault zone

74

where fault-line scarps cut late Plesitocene strata,

but nothing younger (Barrows, 1974).

Besides current tectonic evidence, right-lateral

slip ranging from 915 meters to several kilometers has

been recognized by correlation of offset Pliocene strata

(Hill, 1954; Rothwell, 1958). Barrows (1974) also reports

right-lateral offsets of 180 meters along the Townsite

fault in the Potrero oil field.

Recent studies by Castle and Yerkes (1969) and

Jahns and others (1971) acknowledge right-lateral slip of

nearly 10,000 meters within Miocene, Pliocene, and Pleis­

tocene strata. Harding (1973) has documented right-lateral

offsets of anticlinal axes in the oil fields along the

Newport-Inglewood uplift ranging from 180 meters to

760 meters in post-Miocene time.

Hill (1971) cites several reports of apparent

normal and reverse faulting observed along this zone

(Foster, 1954; Crowder, 1968), but generally discounts

these unfavorable observations as only fault separations,

rather than fault slips, as defined by Hill (1959).

From the results: of this study, Hill's (1971) conclusions

appear to be suspect.

The Charnock fault and the Overland Avenue fault,

which lie west of and parallel to the Newport-Inglewood

fault zone in the northern Los Angeles basin (Figure 3),

are included in this discussion. Both faults are

75

considered active (Association of Engineering Geologists,

1973). Faulted rocks of Late Quaternary age (11,000-

500,000 m.y. B. P.) are present along both faults. No

evidence exists for disturbances since the onset of the

Holocene except for one small fault which displaces

alluvium between the Charnock and Overland Avenue

Faults.

Palos Verdes Hills Fault

The Palos Verdes Hills fault, southwest and sub-

parallel to the Newport-Inglewood fault zone, is a south­

west-dipping reverse fault marked by late Pleistocene

and probable Holocene movement. The onland trace of the

fault lies along the northern flank of the Palos Verdes

Hills uplift. From well data, offsets in Pleistocene

strata mark the latest documented fault movement (Woodring

and others, 1946). Displaced upper and lower Pleistocene

sediments onland, in the city of Torrance, have been

cited in engineering geology reports (Poland and others,

1959) .

Offshore seismic profiling has demonstrated that the

fault continues northward into Santa Monica Bay, in a

segmented fashion, and terminates either at or in the

vicinity of the Dume fault (Junger and Wagner, 1977).

Portions of the offshore northwest extension of the Palos

76

Verdes Hills fault have been mapped by Greene and others

(1975), Junger and Wagner (1977), and Nardin and Henyey

(1978). From published data, this fault appears to

branch along two separate courses (Figure 3). Its inter­

section with the nearly westerly-trending Dume fault is

slightly southwest of Point Dume. In the northeastern

corner of the Santa Monica Bay, Junger and Wagner (1977)

recognize several short, discontinuous, and nearly verti­

cal faults south of the Dume fault which are truncated

by a short, recently-active, and westerly-trending fault

which cuts the base of the Holocene (Junger and Wagner,

1977). This cross-cutting fault appears to mark the

northern truncation of the Palos Verdes Hills fault

extension in the Santa Monica Bay (Ziony and others, 1974).

Nardin and Henyey (1978) and Greene and others (1975)

have also mapped a westerly-trending fault in this vicinity

just to the south of the Dume fault. Both studies show

evidence of fault movement since at least Pleistocene

time.

The southward continuation of the Palos Verdes Hills

fault into the San Pedro basin is marked by Holocene

faulting (Ziony and others, 1974; Junger, 1976; Fischer

and others, 1977; Nardin and Henyey, 1978). From seismic

profiles offshore of San Pedro, Junger and Wagner (1977)

have interpreted gentle warps in the sea floor as either

very late Pleistocene or Holocene.

77

Santa Monica Fault Zone:

East of Beverly Hills

The Santa Monica Mountain structural block has been

uplifted along its southern frontal fault system (the

Santa Monica fault system) since latest Miocene to

early Pliocene time (Yerkes and others, 1965; Yerkes and

Wentworth, 1965; Campbell and others, 1970) , but geologic

evidence does not suggest Holocene displacement except

for its easternmost extensions along the Raymond Hill

and Sierra Madre-Cucamonga faults.

The Santa Monica fault system extends the length of

the study area and for discussion purposes, is divided

into recency of faulting east of and west of Beverly

Hills. East of Beverly Hills, individual faults within

the system are the Santa Monica fault, the Raymond Hill

fault, and the Sierra Madre fault. West of Beverly Hills

is the continuation of the Santa Monica fault, which

bifurcates along the Malibu coast into the Malibu Coast

fault and the offshore Dume fault.

Yerkes and others (1965) describe the orientation and

displacement across the Santa Monica fault at Beverly

Hills. It is a reverse fault dipping approximately 50

to the north. Basement uplift exceeds 2250 meters

(Campbell and Yerkes C19763 note 3650 meters of basement

78

vertical uplift, north side up, indicated in the Beverly

Hills oil field). Basal Miocene strata has been uplifted

approximately 2000 meters (the base of the Modelo Forma­

tion, upper Miocene, is offset vertically a minimum of

450 meters; Lamar, 1961); and the base of the lower Plio­

cene, approximately 900 meters. To the west, upper Plio­

cene and Pleistocene marine deposits are locally warped

and disrupted, but at Beverly Hills and to the east the

base of the upper Pliocene deposits lies unconformably

across the uplifted strata (Knapp and others, 1962).

East of the Beverly Hills, the Santa Monica fault

trace is inferred from oil well data (Robert Hill, pers.

commun., 1978) beneath the alluvium and follows the base

of the Hollywood Hills to the vicinity of the Los Angeles

River (approximately 118° 18' to 118° 10'), where it is

transected by an eight-kilometer-wide zone of northwest-

trending faults which likely is the extension of the

Whittier fault, located farther to the southeast. Lamar

(1961, 1970) has mapped this region of crosscutting

faults. No conclusive evidence exists to suggest

connection of the Santa Monica fault and the Raymond Hill

fault by a fault segment which has demonstrated Quaternary

displacement. Figure 5 shows fault traces, proposed from

field studies in the Los Angeles River flood plain, which

may connect the Santa Monica fault with the Raymond Hill

fault.

79

Between the western banks of the Los Angeles River

and the fault scarp marking the western onset of the Ray­

mond Hill fault, at Arroyo Seco, the northwest-trending

series of complex faults (the Whittier fault extension)

are younger than and offset the east-northeast-trending

Santa Monica fault in a right-lateral sense. In the vici­

nity of the Puente Hills, these faults dip 65° to 75° NE

and have displaced Quaternary strata in a reverse sense,

northeast side up (Woodford and others, 1954), and also

in a right-lateral strike-slip sense (Lamar, 1970). The

Workman Hill fault (Figure 3) is considered potentially

active by the Association of Engineering Geologists

(1973) .

Lamar (1970) has noted that only two northwest-

trending faults display relatively recent geomorphic

features; deflected stream courses and topographic expres­

sion are associated with the San Rafael and Eagle Rock

faults.

The Raymond Hill fault has been active in recent

times. From its western limit at Arroyo Seco, near its

obscurred intersection with the San Rafael fault (Lamar,

1970), to its eastern extent, where it merges with the

Sierra Madre fault (Figure 3), the Raymond Hill fault can

be accurately traced along its 19 kilometer length by a

prominent scarp in the Quaternary surface alluvium

(Bryant, 1978; Yerkes and others, 1965). It is a reverse

80

fault, north side up (1060-1220 meters vertical separa­

tion) along which the crystalline basement complex

(Cretaceous and pre-Cretaceous age) has been juxtaposed

against upper Miocene sedimentary rocks (Yerkes and others,

1965). A steep gravity gradient across the Raymond Hill

fault suggests approximately 200 meters of displacement

in the bedrock, beneath the alluvium (Bryant, 1978).

Latest movement on the Raymond Hill fault has occurred

within approximately the last 200 to 3100 years (Payne

and Wilson, 1974). Evidence for faulting within this time

period includes age dating of carbonaceous sag pond

material and fault gouge at 3100 years and a fresh fault

scarp; small gullies upslope of this scarp show only

minimal erosion. The one sample of crack fill, used in

the age dating, is not conclusively fault gouge. Carbona­

ceous samples in the same area are presently being dated

and should provide more reliability and control on dating

movement along this fault (Robert Hill, pers. commun.,

1978). Bryant (1978) has recently reviewed the Raymond

Hill fault. His review includes a generalized geologic

map showing the location of the Raymond Hill fault

(Bryant, 1978; Plate 1, p. 136-7).

81

Sierra Madre Fault Zone

Only a minor portion of the Sierra Madre fault system

lies wihtin this study area, at its northeastern corner.

The fault is of active status and deserves mention here.

It is part of the east-trending, north-dipping reverse

fault system, which includes the San Fernando fault,

lying along the base of the San Gabriel Mountains where

granitic and metamorphic basement have been thrust upwards

on the north side and juxtaposed against Tertiary and

Quaternary nonmarine deposits (Yerkes and others, 1965;

Jennings and Strand, 1969). Scarps which offset alluvial

deposits are present at several localities along the fault

(Wiggins, 1974); two scarps noted by Lamar and others

(1973) in the Dry Canyon fan near Cucamonga stand six

meters high and 30 meters high, respectively, across the

active fan.

Santa Monica Fault Zone:

West of Beverly Hills

West of Beverly Hills, the Santa Monica fault is

only inferred beneath alluvium from oil company well data

(Yerkes and others, 1965; Wiggins, 1974) and nowhere

displays any recent geomorphic expression (Yerkes and

82

Wentworth, 1965). Evidence for at least lower Pleistocene

fault displacement has been cited by Hoots (1931) across

the mouth of the Potrero Canyon. He has also postulated

up to nine meters of movement along a late Pleistocene

alluvial fan in this vicinity. East of the mouth of

Topanga Canyon, Campbell and others (1966) noted minor

faults displaying a maximum of three meters of offset in

upper Pleistocene deposits and associate them with the

most recent movement along detachment faults active during

Miocene thrusting in the central Santa Monica Mountains.

The problem of assessing the recency of movement along

this segment of the Santa Monica fault system is that

Pleistocene and Holocene deposits are only locally exposed

and rarely well-preserved (Yerkes and Wentworth, 1965).

The Los Angeles Department of City Planning (Wiggins,

1974) classifies this fault as potentially active, showing

no movement since late Pleistocene.

The latest evidence for diastrophsim on the Malibu

Coast fault, from the city of Santa Monica west to the

vicinity of the Point Mugu earthquake, are warped and

disrupted late Pleistocene marine terrace platforms and

deposits (Yerkes and Wentworth, 1965). Yerkes and Went­

worth (1965) note at least seven sites at which late

Pleistocene deposits have been offset with up to at least

four meters of vertical separation. The age of these

marine terrace platforms is bracketed between 35,000

83

years old and 280,000 years old. Recent deposits, which

include nonmarine cover overlying the faulted Pleistocene

terraces, flood-plain deposits, and stream deposits, show

no signs of geomorphic or geologic disturbance (Yerkes

and Wentworth, 1965). Although localized late Pleistocene

fault movement is evident, it represents a relatively

minor period of tectonism when compared with late Miocene

to middle Pleistocene deformation involving displacements

of thousands of meters (Campbell and others, 1966).

The offshore continuation of the Malibu Coast fault,

near the Point Mugu area, is discussed by Junger and

Wagner (1977) . From seismic profiles, they interpret a

zone of scattered faulting with apparent terminations in

Pleistocene sediments to the east of the Hueneme sub­

marine canyon (Figure 2). Most of these faults can be

traced to the surface, where they cut the sea floor. These

segmented faults, shown in Figure 3, are presented as a

broad zone by Junger and Wagner (1977) rather than a

continuous and coherent fault, as is the case onshore.

Greene and others (1975) have recognized a northwest-

trending graben structure on the eastern flank of the Mugu

submarine canyon (Figures 2 and 3) which offsets the

Holocene basement 15 to 25 meters. This graben locates

within the Point Mugu earthquake and aftershock zone.

The Malibu Coast fault is paralleled to the south

along the Santa Monica shelf and slope by the active

84

east-northeast-trending Dume fault which has been mapped

by several authors (Vedder and others, 1974; Ziony and

others, 1974; Greene and others, 1975; Junger and Wagner,

1977; and Nardin and Henyey, 1978) and has been correlated

with the onshore Santa Monica fault. The trace of the

Dume fault presented in Figure 3 approximates the trends

of Vedder and others (1974), Ziony and others (1974),

and Nardin and Henyey (1978), whose maps all agree on its

location. A zone of folded, faulted, and sheared slope

deposits, up to ten kilometers wide involving Quaternary

sediments and upper Pleistocene marine terrace deposits,

lies between this offshore fault and the Malibu Coast fault

to the north (Birkeland, 1972; Vedder and others, 1974;

Ziony and others, 1974; Nardin and Henyey, 1978). The

youngest faulting associated with the Dume fault trace

is recorded in Holocene shelf sediments. Two parallel

west-trending fault traces, from Santa Monica to just

east of Point Dume, are approximately one-half kilometer

apart, nearly continuous, and form a horst which offsets

the Holocene base usually five to ten meters (Greene

and others, 1974; plate 10) .

From a high resolution profile within the vicinity

of Greene and others' interpretation, Nardin and Henyey

(1978; Figure 2B) have identified an unconformity, offset

by the Dume fault trace with the south side down, which

has been interpreted to be approximately 100,000 years

old. 85

FAULT-PLANE SOLUTIONS

General Statement

The fault-plane solutions presented in Figures 16

and 17 are discussed individually, below. The descrip­

tion of each solution includes time of origin of the

earthquake and the strike and dip of the suggested fault

planes. Closed circles represent compressional onsets

and open circles represent dilatational onsets. The

smaller circles represent emergent onsets which are

either more poorly defined first-motion onsets or onsets

lying along the strike of the nodal plane. In a double-

couple earthquake radiation pattern the dislocation along

the fault plane is nil. Therefore, a pattern of emergent

onsets lying along a great circle of the equal-area

projection is suggestive of a nodal plane. This guideline

was frequently used in deciding upon the orientation of

nodal planes. After a set of nodal planes (fault planes)

had been chosen which best fit the data points from a

first-motion plot, then a preferred fault-plane orientation

86

was often determined from known geologic field relation­

ships associated with the causative faults.

Three possible problems limiting the precision of

a fault-plane solution are addressed in Appendix G.

The reliability of a fault-plane solution is dependent

upon the choice of a crustal velocity model. Six possible

velocity models (Figure 9) were used to determine the

fault-plane solutions for five well-located events.

Comparing the resultant fault-plane solutions for each

event, the standard deviation in strike was 36.5 and

dip 27°.

Newport-Inglewood Fault Zone

Fault-plane solutions for the 1973-1976 earthquakes

associated with the Newport-Inglewood fault zone describe

right-lateral strike slip trending north-south and high-

angle reverse slip trending northwest. The preferred

focal mechanism for solutions 1 and 2 (Figures 16 and 20)

and composite solution A (Figures 17 and 20) is right-

lateral strike slip along an approximately vertical fault

striking north-south. Events 1 and 2 are spatially and

temporally related, occurring along the northern portions

of the Charnock and Overland Avenue faults within a

ten day period. Data from these and two other spatially

related events (7505042027 and 761005224; Table 5) when

87

27

0

-c

r*"1

90 2

7°

180

- 90

27

0 -

180

Solution 1:

731129

NS;

72°W

N88°E; 86

S

(N44

°E)

Solution 2:

731209

N4°E; 78IE

N88 W;80 N

(N48°E)

Solution A

N2°E; 90°

N88°W; 80

N

(N45°E)

Figure 20

. Fault-plane solutions 1,

2, and A.

Solution includes ye

ar,

month

day; strike and dip for

both possible fault-plane solutions; an

d orientation of

compressional stress vector (in

parentheses).

00 oo

composited (solution A) lend support to the right-

lateral strike-slip fault mechanism for this portion of

the Newport-Inglewood fault. Focal depths for solutions

1 and 2 are 14 kilometers and 15 kilometers (+ 5 km),

respectively. These focal depths locate within the

basement rocks and support a steeply-dipping fault surface,

suggested by the focal mechanisms.

Fault-plane solution 3 (Figures 16 and 21) and

composite fault-plane solution B (Figures 17 and 21) are

for events which occurred near the location of the 1920

Inglewood earthquake (Richter, 1958) at the southern ter­

mination of the Overland Avenue fault. Solution B is

composed of data points from solution 3 and from a se­

quential event (7311180733; Table 5), triggered approxi­

mately four minutes later. The same focal mechanism

(from solution 3) satisfies the data points for solution

B reverse slip on a northwest-striking fault plane which

dips either to the northeast (32° NE) or to the south­

west (58 SW). The focal depth for solution 3 is 6.9

kilometers (+ 5 km) which does not preclude either fault

plane from consideration.

The epicenter for solution 4 (Figures 16 and 21)

lies to the southeast of epicenter for solutions 1, 2,

and A, on the surface trace of the Charnock fault.

Solution 4 shows reverse slip northwest similar to

solutions 3 and B. The orientation of the possible fault

89

270

- 90

270 -

- 90

270

-

ISO

180

Solution 3:

731118(0729)

N40 W; 32°NE

N50°W; 58 SW

(N50°E)

Solution B

N40°W, 32°NE

N50°W; 58° SW

(N50°E)

Solution 4:

741022(1213)

N24 W; 66

NE

N48°W; 26 SW

(N54

E)

Figure 21

. Fault plane solutions 3,

B,

and 4.

planes are N48° W; 26° SW and N24° W; 66° NE. The calcu­

lated focal depth of 7.1 kilometers (+ 5 km) does not

constrain the choice of fault planes.

Solutions 5 and 6 (Figures 16 and 22) lie west of

the trend of the Charnock fault in the vicinity of the

city of Hawthorne and the Los Angeles International

Airport, respectively. The derived focal mechanism for

both events is reverse slip across similarly dipping

fault planes with orientations of N87 W; 28 S (solution

5) and N51° W; 30° SW (solution 6) or N62° W; 65° NE

(solution 5) and N72°W; 62° NE (solution 6). Either set

of fault planes can accommodate reverse slip associated

with movement along the Newport-Inglewood fault zone.

Focal depths are 10.5 kilometers (+ 5 km) and 9.5 kilo­

meters (+ 5 km), respectively. Projecting hypocenters

for events 5 and 6 on the southwest-dipping fault planes

would realign both events within the Newport-Inglewood

fault zone, at depth.

Composite fault-plane solutions C (Figures 17 and 23),

D (Figures 17 and 23), and E (Figures 17 and 24), com­

piled from earthquakes in the zone of northwest-trending,

en echelon faults and folds associated with the Inglewood

and Potrero faults (with the Newport-Inglewood fault

zone) support the existence of a large component of

reverse faulting in this region. Solution C either

strikes N74 W along a steeply-dipping fault plane

91

270

270 -

180

180

Solution 5:

740117

N62 W; 65 NE

N87°W; 28°S

(N15°E)

Solution 6:

750504(2016)

N51°W; 30°SW

N72°W; 62°NE

(N21

°E)

Figure 22

. Fault-plane solutions 5 and 6.

to

270

-

ISO

ISO

Solution C

N52°W; 32° SW

N74 W 60° NE

(N26

E)

Solution D

N34°W; 44° NE

N34°W; 46° SW

(N56°E)

Figure 23

. Fault-plane solutions C

and D.

10

270

90

270 U

180

-\90

27

0}-

180

180

Solution E

N40°W; 56

SW

N83 W; 44°N

(N17°E)

Solution 7:

730520

N48°W; 70°SW

N66°E; 40°NW

(N9°E)

Solution 8:

761115

N30°W; 48°SW

N73°W; 50°NE

(N39°E)

Figure 24

. Fault-plane solutions E,

7, and 8

(60° NE) or strikes N52° W along a shallowly-dipping

fault plane (32° SW). The shallowly-dipping fault plane

is well defined by emergent P-wave arrivals which, in

turn, defines the auxilary fault plane. Focal depths to

four of the five events composing solution C all plot

between the depths of 8.0 kilometers (+ 5 kilometers)

and 10.5 kilometers (+ 5 kilometers). Data points for

solution D constrain the focal mechanism to nearly pure

reverse slip although the strike may vary up to 26 ,

from N26 W to N52° W. Dips of the possible fault planes

are 44° NE and 46° SW.

Fault-plane solutions 7 and 8 (Figures 16 and 24)

have been combined with four other spatially clustered

events to form composite fault-plane solution E (Figures

17 and 24). All three solutions suggest that reverse

faulting is also present along the southern portion of

the Inglewood and Potrero faults. A minor component of

right-lateral strike-slip displacement is present on the

preferred fault plane which strikes to the northwest and

dips to the southwest. The dips on the various faults

range from 48° SW to 70° SW. The data points from the

four additional events (7503270652, 7510270207, 7511250236,

and 7609020256) comprising part of solution E are also

plotted in Figure 24. The preferred focal mechanism

for these combined earthquakes is also reverse faulting

with a minor component of right-lateral strike-slip

95

displacement (N40 W; 56° SW). Two events from solution E

have reliable focal depths computed at 15.9 and 8.0

kilometers (+ 5 kilometers), which is consistent with

focal depths computed for other earthquakes associated

with this zone of deformation.

Focal mechanisms for solution 9 (Figures 16 and 25)

and composite solution F (Figures 17 and 25), derived from

a cluster of seven events near the cities of Paramount

and Lakewood (Figure 3) demonstrate reverse slip with a

large component of strike-slip. Reverse faulting, also

present to a significant degree in ten of the thirteen

fault-plane solutions (solutions 3, 4, 5, 6, 7, 8, B,

C f D, E) presented for the Newport-Inglewood fault zone,

appears to be as common and important a faulting mechanism

as right-lateral strike-slip faulting. Unlike these same

solutions (solutions 3, 4, 5, 6, 7, 8, B, C, D, E), the

data points for solutions 9 and F do not suggest a north­

west-trending fault plane. For both solutions 9 and F,

right-lateral strike-slip motion is accommodated as a

component on a reverse fault oriented N86 w; 38 S

or left-lateral strike-slip motion is accommodated on ao 0

reverse fault oriented N53 E; 60 NW. Thus, neither

solution can be preferred to fit the pattern of faulting

developed from the previous fault-plane solutions associated

with the more northerly portion of the Newport-Inglewood

fault zone.

96

270

^9

0

27

0

180

180

180

Solution 9:

760715

N53 E; 60 NW

N86°W; 40°S

(N16°W)

Solution F

N.53°E;

60°NW

N86 W; 40 S

(N16°W)

Solution 10:

760627

N68 W;

50

NE

N80°E; 54°SE

(N15°E)

Figure 25

. Fault-plane solutions 9,

F,

an

d 10

.

vo

Solution 10 (Figures 16 and 25) occurred to the east

of the Newport-Inglewood fault zone and south of the Santa

Monica fault zone, within the Los Angeles city limits.

The event is not associated with any known active fault.

The fault-plane solution indicates reverse slip on a

north- or south-dipping fault plane oriented either N68 W;

50° NE or N80° E; 54° SE.

Fault Plane Solutions; Santa Monica Bay

Offshore, in the Santa Monica Bay, six fault-plane

solutions, derived from events occurring along faults

associated with the northwest extension of the Palos

Verdes fault (Figures 16 and 17), reveal that fault move­

ments have probably been restricted to fault planes trend­

ing northwest.

Composite fault-plane solution G (Figures 17 and 26),

derived from a cluster of three earthquakes (events

7303292135, 7503071629, 7509271247), locates just south

of the Santa Monica fault, where it extends offshore to

meet the Dume fault, and slightly east of the northwesterly-

trending fault which intersects the Dume fault, as mapped

by Junger and Wagner (1977). Choosing between the two

possible fault planes, movement has most likely occurred

in a normal sense along a plane striking N42° W and dipping

79° NE. This fault plane parallels the northwest-trending

fault but is inconsistent with its displacement (east 98

270

90

270

180

Solution G

N42 W; 79 NE

N9 W: 11 W

(N26°W)

90

27O

-

180

Solution H

N41°W; 60°NE

N85 W; 40°S

(N27°E)

Solution 11:

751007

N46 W; 40 SW

N61'°W;

50°NE

*7C

(N37

°E)

Figure 26

. Fault-plane solutions G, H,

and 11

VO

VO

180

Solution I

N50°E; 60°SE

79°NE

(N2E)

side up) as mapped by Junger and Wagner (1977). Since

this fault is truncated by the dominant east-west trending

Santa Monica-Dume fault system, constraints are imposed

on possible strike-slip movement. Consistent with this

condition, the preferred fault plane exhibits only a minor

component of right-lateral strike slip.

To the south, focal mechanisms for composite fault-

plane solutions H and I (Figures 17 and 26) suggest the

presence of an increased right-lateral strike-slip component

of displacement with an increased distance, south, away

from the east-west trending Dume fault. Solutions G,

H, and I have a common fault plane striking approximately

northwest and dipping steeply to the northeast (dips of

79°NE, 60°NE, and 79°NE, respectively). This fault plane

orientation is contrary to the southwest dip of the onland

Palos Verdes fault, further to the south. This same phenon-

menon has been observed by Terry and others (1956) in an

earlier interpretation of the Palos Verdes Hills fault.

They suggest the fault changes from a steep southwesterly

dip onland to a steep northeasterly dip in the offshore

bay area.

Further south in the Santa Monica Bay, solution 11

(Figures 16 and 26) describes reverse faulting with a

minor component of strike-slip displacement. Although

motion on either fault plane is possible, the fault plane

striking N46° W and dipping 40° SW also accommodates

100

right-lateral strike-slip displacement and agrees with

the orientation of the onland portion of the Palos Verdes

fault.

A sequence of four earthquakes occurred on November 22,

1976 in the Santa Monica Bay and trend northwest on the

northern flank of the Santa Monica submarine canyon across

a portion of the Palos Verdes Hills fault extension, which

has moved most recently during the late Pliocene (Figure 2;

Junger and Wagner, 1977). The location of these events is

outlined in Figure 17 by composite fault-plane solution J

(Figure 27). Fault-plane solution 12 (Figures 16 and 27)

and solution J both define right-lateral strike-slip

motion on a steeply-dipping fault plane which strikes north-

northwest. The preferred fault plane for solution 12 is

N10° W? 70° W. The preferred fault plane for solution J

is N20° W; 80° NE. The data points from solution 12 are

incorporated as part of composite solution J.

The sequence of four epicenters align along a trend

striking N15°W which also supports the above preferred

solutions. The alternative fault planes are both left-

lateral strike-slip trending approximately east-northeast,

parallel to the Dume fault trace, three kilometers to the

north. The location and possible focal mechanisms for

solutions J and 12 are similar to solutions A, 1, and 2,

which occurred on the Newport Inglewood fault, only

several kilometers south of the Santa Monica fault.

101

270

ieo

Solution J:

N20 W; 80 NE

N73°E; 78°SE

(N26°E)

270

180

Solution 12

: N1

0°W;

70 W

N80°

E; 90

°

(N35

°E)

Figure 27

. Fault-plane solutions J

and 12

.

o

to

South of this cluster, Nardin and Henyey (1978) have

identified a west-northwest-trending fault along the

southern flank of the Santa Monica submarine canyon which

displaces late Quaternary strata, north side down. This

fault dips less than 30 and has been associated with a

low-angle slump surface. It is unlikely that this fault

was responsible for the strike-slip movement associated

with the earthquake sequence to the north.

Further west in the Santa Monica Bay, but east of the

Dume anticline, twenty earthquakes align along a broad

northwest trend, parallel to the San Pedro escarpment

(Figures 3 and 12). Portions of the San Pedro fault zone

have moved since Quaternary time, but most of its length

has not been active since the Pliocene (Junger and Wagner,

1977). Although seismic activity is high in this portion

of the bay, no active faults have been mapped here. The

quality of data decreases away from the coast due to less

seismic station control over greater distances. Due to

this circumstance, only two reliable fault-plane solutions,

13 and 14 (Figures 16 and 28), have been derived from events

occurring in the outer Santa Monica Bay. Although neither

solution is conclusive, both suggest reverse faulting with

a minor component of strike slip.

103

270

2TO -

180

180

Solution 13

: 730307(2349)

N55°W; 50°SW

N79 E; 40°N

(N12°E)

Solution 14:

750123

N61°E; 50

NW

N78°E; 40°SE

(N29

W)

Figure 28.

Fault-plane solutions

13 an

d 14.

Fault Plane Solutions:

Santa Monica Fault , East of Beverly Hills

Four fault plane solutions (15, 16, 17, and 18; Figures

16 and 29) align along the surface traces of the Santa

Monica fault between Beverly Hills and Hollywood. In

the Hollywood-Los Angeles area, the east-northeast trace

of the Santa Monica fault is transected by a zone of

northwest-trending faults along the projection of the

Whittier and Workman Hill fault (Figures 3 and 5). Three

additional fault-plane solutions (19, 20, and 21; Figures

16 and 30) have been derived from earthquakes occurring

along this northwest-trending zone of transecting faults,

south of the Santa Monica-Raymond Hill fault zone.

Contrary to northwest-trending focal mechanisms for

earthquakes occurring south of the Santa Monica fault

system, specifically along the Palos Verdes fault exten­

sion in the Santa Monica Bay and along the Newport-

Inglewood fault zone, fault planes associated with the

Santa Monica fault zone generally trend to the northeast.

This trend coincides with the east-northeast trend for

the surface trace of the Santa Monica fault in this region

reverse fault displacement is the dominant fault mechanism

for all four solutions (15, 16, 17, and 18). The preferred

fault pl-ane for solution 18 is N79°E; 64° N and agrees

105

270

- »o

2ro

-2TO

Solution 15:

760929

N6°E

; 70°W

N36 E;

24°SE

(N76°W)

Solution 16:

740312(0735)

N29°E; 12°NW

°

N29°E; 78°SE

(N61

°W)

Figure 29

Solution 18:

741206

N49 E;

30°SE

N78°E; 64°N

(N26

°W)

Fault-plane solutions

15,

16,

17,

and

18.

270

180

Solution 17:

740306

N21°E; 50°NW

N53 E; 44°SE

(N53°W)

teo

2 TO

90

270 -

270

180

180

I8O

Solution 19

: 751011

N50°W; 72°SW

N55°E; 48

NW

(N2°

E)

Solution 20

: 741219

N42°W; 56°NE

N81 E;

50

S

(N61

E)

Solution 21

: 740227

N44 W; 80

SW

N76 W;

16°N

(N30°E)

Figure 30

. Fault-plane solutions 19

, 20

, and 21.

with north-over-south reverse faulting common along the

southern frontal fault system of the Santa Monica Mountains.

The fault planes for the other three solutions trend more

northerly than the trace of the Santa Monica fault zone.

The preferred fault plane for solution 17 is N21° E;

50 NW. A preferred fault plane for solutions 15 and 16

is indeterminable with respect to the Santa Monica fault

zone.

No fault-p^ane solutions have been derived for events

on or north of the tectonically active Raymond Hill fault.

South of the Raymond Hill fault, three solutions (19, 20,

and 21; Figures 16 and 30) spatially align within the zone

of northwest-trending faults projecting north from the

Whittier fault zone. The focal mechanism for solutions 20

and 21 is reverse slip on preferred fault planes striking

northwest, parallel to the trend of the fault zone. For

solution 20, the fault plane oriented N42° W; 56° NE also

exhibits a component of right-lateral strike-slip displace­

ment (besides reverse displacement) which agrees with known

displacement along the Whittier fault system. Solution

21 shows only reverse displacement most likely across a

steeply-dipping fault plane oriented N44 W; 80 SW.

The possible fault planes for solution 19 satisfy fault

displacements for both the northwest-trending fault zone,

described above, and the northeast-trending Raymond Hill

fault. The northwest-trending fault plane is oriented108

N50° W; 72° SW and accommodates right-lateral strike-

slip faulting with a component a reverse fault displace­

ment. The alternative fault plane, N55° E; 48 NW, is

consistent with the known orientation of young geologic

offsets on the Raymond Hill fault but lies south of its

surface trace.

Solutions 19 and 20 each have several data points

which do not fit the derived fault plane solutions and

therefore are less reliable as unique solutions as compared

to other solutions in this study. Both are presented here

as the best representative fault-plane solutions from the

existing data set (1973-1976 earthquakes) for this

complex area. As discussed by Lamar (1970) and others,

two fault systems with opposing trends (the Santa Monica

fault system and the Whittier fault system) intersect in

the Los Angeles River floodplain area, within the study

area. At the intersection, crosscutting fault relationships

are obscured by alluvium. Well-controlled fault-plane

solutions would define present-day movement between these

two fault systems and lead to an improved tectonic under­

standing of this complex geometry. Solutions 15, 16, 17,

and 18 reveal no evidence for right-slip intervention (due

to the Whittier fault system) west of the intersection zone

of faults. The two solutions to the south (20 and 21)

describe reverse and right-lateral strike-slip displace­

ment on a fault plane striking northwest, which suggests

109

270

90

3 TO [

-

190

Solution 22

: 741106(0038)

N36°E; 54°NW

N36°E; 36°SE

(N36°E)

ieo

Solution K

N14°W; 58°E

N56°E; 60°NW

(N21°E)

Figure 31

.

Solution 21

: 740331

N74°W; 30°SW

N88°E; 60 N

(N82

°W)

Fault-plane solutions 22,

K, 23,

and 24.

270

270-

160

Solution 23:

760620

N46°W; 60°SW

N71 E; 50°NW

(N12

°E)

that the Whittier fault zone extension is the causative

fault. Closer to the fault zone intersection, solution

19 describes two possible fault planes which can accommo-

data displacement occurring on either fault system.

Associated with the Sierra Madre fault, in the north­

eastern corner of the study area, fault-plane solution 22

(Figures 16 and 31) describes normal displacement across

a fault plane striking N36 E and dipping either 54 NW

or 36° SE. Normal displacement for this event is contrary

to the north-dipping reverse fault documented along this

fault zone. Since only one solution is available on this

isolated portion of the fault (in this study area) no

conclusions have been drawn as to its consequences.

Fault Plane Solutions:

Santa Monica Fault Zone, West of Beverly Hills

In the northwestern Santa Monica Bay, within the

Malibu Coast zone of deformation, focal mechanisms for

solutions K and 23 (Figures 16, 17, and 31) show a large

component of left-lateral strike-slip displacement and

solution 24 (Figures 16 and 31) describes normal fault

displacement. The preferred fault planes strike to the

east and northeast. Even though the fault planes parallel

the trend of the Santa Monica fault system, none of these

solutions can be spatially assigned to a probable causative

fault.

This significant component of left-lateral strike-slip

displacement (from solutions K and 23) is also suggested

by the alternative focal mechanisms for five solutions

(1, 2, A, 12, and J; Figures 16 and 17) which all locate

just south of the Santa Monica fault. Although the

reverse fault mechanism appears dominant along the Santa

Monica fault system, several solutions (1, 2, A, 12, J,

23, and K) suggest that left-lateral strike-slip displace­

ment may also be an active component of faulting along

and directly south of this fault system.

In the Santa Monica Mountain structural block, two

fault-plane solutions are isolated and cannot be inter­

preted without additional supporting data. Solution 25

(Figures 16 and 32) defines normal fault movement along

a northwest-striking plane. Its location is approximately

five kilometers north of the Malibu Coast fault and five

kilometers east of the Simi fault extension. The possible

faults dip 70° SW and 20° NE.

The only other fault-plane solution, 26, (Figures 16

and 32) locates southwest of the Northridge Hills fault.

The possible focal mechanisms are N48 W; 75 NE, reverse-

slip with a large component of right-lateral slip or

N60° E; 43° SE, left-lateral strike-slip.

112

270

2TO

ISO

180

Solution 25

: 760504

N48 W;

20 NE

N48 W;

70°SW

(N48°W)

Solution 26

: 750128

N48IW; 75°NE

N60°E; 43°SE

(N6°

E)

Figure 32.

Fault-plane solutions 25 an

d 26

.

SEISMICITY PATTERNS, MAGNITUDES, AND FOCAL DEPTHS

The seismicity patterns and fault-plane solutions

derived from four years of data (1973-1976) suggest that

present-day tectonic activity is strongly controlled by

several pre-existing major fault zones. Diffuse trends

of seismicity align parallel, in a northwesterly direction,

south of the Santa Monica fault system (Figure 33). Two

trends coincide with the Newport-Inglewood fault zone and

the Palos Verdes Hills fault extension in the Santa Monica

Bay. The third diffuse trend begins north of Point Dume

and projects southwestward from the Malibu Coast fault

into the Santa Monica Bay, just east and parallel to the

San Pedro basin fault zone (Figure 3). No mapped faults

are associated with this trend.

Within the study area, the most concentrated group of

seismic events occurred as a sequence of aftershocks

subsequent to the February 21, 1973 Point Mugu earthquake,

located along the westernmost mapped portions of the

Malibu Coast fault and extending south, offshore, to the

Dume fault (Figure 3). The areal extent of this sequence

114

Figure 33. Outline of seismicity trends.

115

closely corresponds to the Malibu Coast zone of deformation

as described by Yerkes and Wentworth (1965) to define the

approximately 10 kilometer wide zone of Quaternary defor­

mation south of the Malibu Coast fault and north of the

Dume fault.

East of Point Dume, only one earthquake cluster is

associated with the Santa Monica fault system. The cluster

straddles the fault in the Hollywood area, west of the

probable intersection with the Whittier fault zone. Be­

tween Point Dume and Hollywood, there is a noticeable

lack of seismicity on the Santa Monica fault and to the

north, within the Santa Monica Mountain block. The diffuse

seismicity trends associated with the Newport-Inglewood

and the Palos Verdes Hills fault zones terminate just

south of the Santa Monica fault zone. The north-dipping

Santa Monica fault is poorly delineated by the scattered

epicenter distribution in the Santa Monica Mountain block.

The mountain block has been seismically less active (east

of Point Dume) than the Newport-Inglewood and Palos

Verdes Hills faults. This lack of seismicity associated

with the Santa Monica Mountains has also been documented

by Lee and others (1979) for the western Santa Monica

Mountains (1970-1975 earthquakes) and by Real (1979)

for the eastern Santa Monica Mountains (1972-1975 earth­

quakes) . Since the throughgoing Santa Monica fault

terminates the northwest-trending structures and also

117

appears to be the northern boundary for most of the seis-

micy, part of the crustal shortening may presently be

accommodated to the south as folding within the north­

west-trending basin structures, as well as thrusting on

the Santa Monica fault zone. From the distribution of

epicenters, the Santa Monica fault and the northwest-

trending faults, including those in the Santa Monica Bay,

appear to be the major components of local tectonic defor­

mation. Faults north of the Santa Monica fault have

been seismically less active, thus the Santa Monica

Mountains appear to remain as a relatively passive and

coherent structural block.

The Newport-Inglewood seismicity trend aligns closely

with the defined fault zone in the northwestern portion

of the Los Angeles basin. The preferred vertical fault

planes defined by solutions 1, 2, and A (Figure 20)

agree with this alignment. An apparent concentration

of events within the Baldwin Hills area represents a

bias due to the BHSN's local station coverage.

Further to the southeast, earthquake epicenters continue

to parallel the northwest-southeast trend of the Newport-

Inglewood fault zone, but group slightly east of the Cherry

Hill and Seal Beach faults (Figure 12). Ziony and others

(1974) describe a northwest-trending fault in this epicen-

tral area, near the cities of Paramount and Lakewood

(Figure 3). Evidence of faulted early Pleistocene

(2,000,000 to 500,000 years B.P.) marine terrace 118

deposits and unfaulted late Pleistocene (500,000-11,000

years B.P.) deposits bracket the latest movement along

this fault. The fault is buried beneath Recent deposits

and is largely inferred from ground water reports. Seven

epicenters, between 1973 and 1976, align along the fault

trace and extend beyond its northwest termination, as

determined by Ziony and others (1974) . If the trend of

these epicenters is associated instead with the Newport-

Inglewood fault zone, the data would suggest a steep north­

easterly dip on the Seal Beach and Cherry Hill faults.

Neither focal mechanism (Solutions 9 and F; Figure 25),

which have been derived from this cluster, support a

northeasterly-dipping fault plane.

On the Santa Monica shelf, seismicity has been concen­

trated along the length of the Palos Verdes Hills fault

extension rather than the more precisely defined Redondo

Canyon fault and the Palos Verdes Hills fault, to the

south. Seismicity associated with the Whittier fault zone,

the Sierra Madre fault and the central Los Angeles basin

is marked by only isolated events.

The Point Mugu earthquake, which registered a magnitude

of M = 5.9 in this study (Ellsworth and others C19733 esti­

mated M = 6.0), and five other events have occurred of

noteworthy magnitude (Figure 13). A sequence of two events

(741210C12363 and 741219C12393); Appendix D) with magnitudes

M = 4.3 and 3.8, respectively, occurred in the vicinity

119

of the Whittier-Santa Monica fault intersection. Along

the same trend and to the northwest another we11-located

event registered M = 4.3 (741206; Appendix D). Several

other events with magnitudes 3<M<4 cluster in this region

and mark this zone as one of significant activity. These

three events have a focal depth of approximately 8 to 10

kilometers (+ 5 km). Another sequence of two earthquakes

(761122C17533 and 761122C17553; Appendix D) with magnitudes

both greater than 4.0 occurred along the Palos Verdes Hills

fault extension just south of the Dume fault. Similar to

the above earthquakes, this sequence occurs at the inter­

section of the two opposing structural regimes. One final

event (750512; Appendix D) registers M = 4.8 but is only

determined from one duration reading and consequently is

suspect.

In general, magnitude 3 or greater earthquakes have

grouped along the Santa Monica fault in the Hollywood

area; along the Whittier fault, southwest of the Holly­

wood cluster; along the southern half of the Newport-

Inglewood fault zone; and in the Santa Monica Bay near both

ends of the Palos Verdes Hills fault trend where it

intersects the Dume fault and the Redondo Canyon fault,

respectively. This pattern suggests a build-up and release

of stress concentrated at the intersections of these cross-

cutting fault zones.

120

Focal depths for relocated "A" and "B" quality earth­

quakes are generally deeper than eight kilometers but typi­

cally less than 12 kilometers. Tectonically, this deep

hypocenter bias supports the hypothesis that most of the

displacement south of the Transverse Ranges is being

accommodated on deep northwest-trending basement faults;

this movement is often expressed at the surface as uplifted

intervening anticlines.

121

STRESS REGIME AND PREFERRED FAULT-PLANE SOLUTIONS

From the stress vectors derived in this study (Figures

18 and 19), the Los Angeles basin and the continental

borderland of coastal southern California presently appear

to be responding to a regional northeast-southwest- to

NNE-SSW-trending compressional stress regime. Earthquakes

spatially associated with the Newport-Inglewood fault zone

and the northwest-trending faults in the Santa Monica Bay

predominantly describe stress vectors with this orientation.

Similar seismic studies by Yerkes and Lee (1979) and Real

(1979) also describe overall compressional stress axes

approximately normal to the strike of the San Andreas

fault (N24°E). This dominant stress pattern is complicated

by an apparent northwest-southeast compressional stress

localized along the Santa Monica fault near its intersec­

tion with the northeast extension of the Whittier fault

zone. From fault-plane solutions, this local change in

the stress regime may possibly be caused by geometrical

constraints imposed on fault interactions at the inter­

section of these two opposed structural trends.

122

The four events (15, 16, 17, and 18; Figure 16) defining

northwest-southeast compression have high-angle reverse-

fault mechanisms which may be interpreted in terms of north-

over-south thrusting across the Santa Monica fault. The

extension of the Whittier fault zone intersects the Santa

Monica fault to the east of these four solutions. Events

20 and 21 (Figure 16) suggest seismic activity along the

Whittier fault system and fault-plane solution 20 supports

the notion of right-lateral strike-slip fault displacement.

The existence of a local northwest-southeast compressional

stress isolated to the west of the fault intersection also

supports the notion that right-lateral strike-slip dis­

placement is actively occurring on the Whittier fault

extension. In terms of the fault intersection geometry,

right-lateral strike-slip along a northwest-trending fault

will create a northwest-southeast compressional stress on

the western side of an intersection fault (the Santa Monica

fault) .

Solution 19 (Figure 16), close to the intersection of

these crosscutting faults, is transitional in that the

possible focal mechanisms described by this solution can

fit fault movement along either fault system. The stress

information is also transitional, with a compressional

stress vector oriented N2°E. Therefore, neither fault can

be pinpointed as the more likely causative fault. This

transitional solution and stress orientation suggests a

123

region of interaction at the intersection of the Santa

Monica fault and Whittier fault extension.

Besides, evidence for high-angle reverse faulting

associated with the Santa Monica fault zone, several events

aligning along the northern boundary of the Los Angeles

basin (solutions 1, 2, 12, 23, A, J, and K) demonstrate

a possible component of left-lateral strike-slip displace­

ment, consistent with the regional compressional stress

field. Supporting this sense of displacement is a fault-

plane solution from five earthquakes (1972-1976) associated

with the Santa Monica-Raymond Hill fault zone which des­

cribes oblique left-lateral reverse slip (Real, 1979).

West of the study area, fault-plane solutions for earth­

quakes associated with the Anacapa fault (the westward

extension of the Dume fault) show north-over-south reverse

slip, consistent with mapped late Quaternary fault displace­

ments in the Santa Barbara Channel (Yerkes and Lee, 1979).

Slip vectors analyzed by Yerkes and Lee (1979) for 51

events (1970-1975) demonstrate that the western Transverse

Ranges have been dominated by oblique left-lateral reverse

slip.

Three of the four normal fault-plane solutions (solu­

tions G, 24, 25; Figures 16 and 17) from this study group are

in the vicinity of the Malibu Coast zone of deformation.

The only other normal fault-plane solution (solution 22;

Figure 16) is on the Sierra Madre fault. Extensional

124

faulting and left-lateral slip are isolated south of the

western Santa Monica Mountains. These fault mechanisms

may be due to a clockwise rotation of the east-west-trend­

ing structures in response to right-lateral strike-slip

motion regionally imposed by the San Andreas fault and

locally expressed at the intersection of the opposite-

trending structural regimes. The lack of seismicity within

the Santa Monica Mountains relative to the concentration of

seismicity to the south (Figure 12) suggests that the

western Santa Monica Mountains may be decoupled from the

Santa Monica basin and are rotating clockwise as a passive

and coherent structural block. Paleomagnetic evidence

indicates that the Santa Monica Mountains have previously

undergone clockwise rotation. Kamerling and Luyendyk

(1979) have analyzed paleomagnetic data from Miocene vol­

canic rocks which suggest that the Santa Monica Mountains

have been rotated clockwise approximately 70° and translated

northward approximately 10 latitude since middle Miocene

time. They suggest clockwise rotation has occurred about

a point in the eastern Santa Monica Mountains. Both left-

lateral and normal faulting can be accommodated by clock­

wise rotation. According to a tectonic model derived by

Crouch (1979), the translation and rotation of the western

Transverse Ranges could have been largely completed by

8 m.y. B.P.

125

Other than these isolated normal fault-plane solutions,

the region has been dominated by compressional tectonics.

The 1973 Point Mugu earthquake generated north-over-south

reverse slip with a lesser component of left-lateral

strike slip (Ellsworth and others, 1973). The preferred

fault-plane orientation is N69°E; 49°N. Location of

the Point Mugu earthquake (Ellsworth and others, 1973)

and its aftershocks (Steirman and Ellsworth, 1976) support

the notion derived from the seismicity patterns that inter­

action between the juxtaposed structural provinces is con­

fined to a transition zone known as the Malibu Coast zone

of deformation. Lee and others (1979) have relocated the

1973 Point Mugu earthquake sequence and conclude that the

Anacapa fault, a western extension of this zone of deforma­

tion, was the causative fault.

For earthquakes south of the Santa Monica fault zone,

the preferred focal mechanisms mostly describe high-angle

reverse faulting with right-lateral strike-slip as a

secondary component of displacement. Reverse faulting is

not readily recognized from field data along the Newport-

Inglewood fault zone, whereas right-lateral strike-slip

offsets are commonly described in the shallow strata and

by offset en echelon anticlines, common to this zone

(see Latest Faulting: Newport-Inglewood Fault Zone). The

focal mechanism descriptions for 1973-1976 earthquakes

occurring along the northwest-trending faults support

126

the concept of small-displacement convergent right-lateral

wrench faulting (Harding, 1973; Wilcox and others, 1973;

Junger, 1976). Junger (1976) and Wilcox and others (1973)

have performed claycake experiments, simulating right-

lateral wrench faulting in the continental borderland

and the Newport-Inglewood fault zone, which demonstrate

that significant amounts of vertical displacement can be

accomplished by folding as a result of only limited dis­

placement due to faulting at depth (Garfunkel, 1966).

Hypocenters along the Newport-Inglewood fault zone group

at depths of approximately eight to ten kilometers. Base­

ment faults may be the source of high-angle reverse focal

mechanisms described herein. The northwest strike of the

fault planes is in accordance with the described compres-

sional stress regime oriented NNE-SSW.

Although the preferred focal mechanisms for the Newport-

Inglewood fault zone suggest northwest-striking fault

planes, the data does not describe a consistent dip for

this fault zone. Preferred fault planes dip vertically as

well as to the northeast, southwest, and south. Real

(1979) proposes that the anastomosing nature of fault

strands in the fault zone are capable of producing strike-

slip, normal, and reverse fault mechanisms. From five

earthquakes in this zone, Real has composed a fault-plane

solution which shows right-lateral strike-slip accompanied

by a secondary component of normal dip slip. The fault

127

plane dips steeply to the southwest. A similar solution

derived by Teng and others (1977) describes a fault dipping

steeply to the northeast.

Recent marine geophysical studies by Nardin and Henyey

(1978), Junger and Wagner (1977), and Junger (1976) also

support the concept of convergent right-lateral wrench

faulting for the continental borderland. Vertical uplift

in Pliocene and Quaternary time appears not to have been

concentrated along the right-lateral wrench faults but

rather along the intervening en echelon northwest-trending

folds occurring within the continental borderland and the

Los Angeles basin.

128

CONCLUSIONS

Within the study area, field studies describing geo­

logic and geomorphic evidence for Holocene movement reveal

the Raymond Hill fault, the Sierra Madre fault, and the

Charnock and Overland Avenue faults (within the Newport-

Inglewood fault zone) as active faults. Microseismicity

during 1973-1976 is spatially associated with the surface

traces of the Charnock and Overland Avenue faults as well

as with other fault segments comprising the northwestern-

most portion of the Newport-Inglewood fault zone. The

associated microseismicity supports the active status

for this fault zone. The Sierra Madre fault was the locus

of three earthquakes during 1973-1976 while the Raymond

Hill fault was inactive during the study period.

Marine geophysical seismic profiles interpreted by

Greene and others (1975) describe offset Holocene shelf

sediments in the vicinity of the 1973 Point Mugu aftershock

zone and to the east, on the Dume fault trace. The 1973

Point Mugu earthquake, its aftershocks and subsequent

minor activity through 1976, cluster along and just north

129

of the Malibu Coast zone of deformation and account for

the largest concentration of seismic events located in

this study. This seismicity confirms the western portions

of the Santa Monica fault zone as active.

Several potentially active faults (defined by most

recent fault displacements dating between 11,000 years

B.P. and 2,000 000 years B.P.) should be classified as

active since microseismicity appears to have been caused

by these faults. These faults include the intersecting

Santa Monica fault and the northwest extension of the

Whittier fault, in the vicinity of the Los Angeles River

floodplain, and the offshore northwest extension of the

Palos Verdes Hills fault, on the Santa Monica shelf. Two

clusters of seismic activity have occurred during 1973-1976

which are not spatially associated with any active or

potentially active faults. These clusters occur northeast

of the Cherry Hill and Seal Beach faults of the Newport-

Inglewood fault zone and in the western Santa Monica Bay.

Noticeable seismicity gaps during 1973-1976 exist along

fault segments which previously have been subjected to

earthquake occurrences or which show Pleistocene and Holo-

cene diastrophism. These faults are the Santa Monica

fault, particularly west of Beverly Hills and east of

Point Dume; the Raymond Hill fault; the Verdugo fault; the

onland portion of the Palos Verdes fault; and the Redondo

Canyon fault. Seismic activity is concentrated in the Los

130

Angeles basin and Santa Monica Bay rather than within the

Santa Monica Mountain block. The number of seismic events

terminates at the Santa Monica fault system rather than

continuing to the north, along the projected trend of the

northwest-trending Newport-Inglewood fault zone and the

Palos Verdes Hills fault extension. This sharp seismicity

decrease corroborates structural evidence which differen­

tiates the Santa Monica Mountains as a separate struc­

tural entity from the basins to the south.

Contemporaneous deformation and Neogene evolution of

these juxtaposed structures appear to be responding to a

regional northeast-southwest compressional stress regime

which has been derived from fault-plane solutions for

1973-1976 earthquakes. A combination of high-angle reverse

faulting with secondary right-lateral strike slip has been

the major deformational mode south of the Santa Monica

fault system. Although only insignificant amounts of

right-lateral strike-slip have been documented on the Palos

Verdes Hills fault northwest extension (Junger and Wagner,

1977), two fault-plane solutions describe a component of

right-lateral strike-slip taking place on the Santa Monica

shelf.

Concentrations of seismicity, associated with the

Newport-Inglewood fault zone and the Palos Verdes Hills

fault estension, appear to be originating at depth within

the crystalline basement (hypocenters>10 kilometers).

131

From geologic field evidence, reverse displacement has

not been recognized as the major component of faulting,

particularly along the Newport-Inglewood fault zone. This

reverse displacement is occurring at depth and apparently

does not continue to the surface along definitive fault

planes. Rather, this motion may be translated into folding

of the younger structures associated with these basin

fault zones.

Fault-plane solutions along the northern Los Angeles

basin suggest that movement across the east-northeast-

trending Santa Monica fault system is in a general north-

over-south reverse sense accompanied by a secondary com­

ponent of left-lateral strike-slip displacement. This

secondary component of motion is consistent with the

expected fault movement in response to a regional northeast-

southwest compressional stress regime.

Isolated on the eastern portion of the Santa Monica

fault, in the Hollywood area, are reverse fault mechanisms,

striking northeast-southwest, in response to a local

northwest-southeast-oriented compressional stress field.

This isolated northwest-southeast compressional stress

is most likely due to geometrical constraints created

at the intersection of the east-northeast trending Santa

Monica fault and the northwest-trending Whittier fault.

Three normal focal mechanisms have occurred along the

Malibu Coast zone of deformation and may be in response

132

to a regional clockwise rotation of the Transverse Ranges,

motivated by right-lateral strike slip along the San

Andreas fault. These normal solutions suggest decoupling

between the Transverse Ranges and the northwest-trending

structural provinces to the south.

133

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141

APPENDIX A: List of seismic stations used for recording and relocating 1973-1976 earthquakes.

142

Table 6. List of seismic stations used for recording and relocating 1973-1975 earthquakes. The three DELAY columns represent successive refinements in the staion delay constant (in seconds) as applied to each correspond­ ing station. The last column represents the correction factor used in the final reloca­ tion for 1973-1975 earthquakes. Asterisk indicates USC station.

143

1234567891011121314151617181920212223242526272829303132333435363738394041424344454647

STN

PNMJASSAGPRIMNVMHCEHCBLUCKCSHELEDSDWCKCDB2KEECPEIRNCPMWWRMLLDVLBC2C02SHHINSRMRBMMCFT

WINDTPNGSYCASQPTSIMISHERSBOBPOTRNIDOMUGULONGLEMNEGGRDUMBDEERCLEFCAMHAGORTRP

LAT

3850.3756.3645.36 8.3825.3720.4048.3424.34 8.3349.3428.3436.34 8.3344.3338.3252.34 9.34 9.3359.34 5.3411.3339.3350.3411.3356.3412.3345.34 2.34 5.34 6.34 8.34 3.3414.34 7.34 2.34 9.34 3.34 6.3419.3410.34 7.34 0.34 4.3414.3415.34 0.34 4.

84NSON90N50N93N50NION32N18N36N06N55N18NION30NSON60N24N5 IN48N99N42N83N26N14N77N40NUN15N47N37N37N8 IN44N20N37N45N87N77N79N87N25N02N07N19N52N33N

LONG

12256.12026.12126.12039.118 9.12138.12359.11743.11710.11721.11556.117 4.11710.117 3.11639.117 6.11511.11611.11639.11656.11719.11527.11520.11539.11611.11634.11435.117 6.119 2.11836.11858.11855.11848.11851.119 1.11859.11840.119 4.11856.119 1.119 8.11848.11859.11854.119 2.11844.11834.

78W30W70W90W26W50Wlow52W48W32W19W45W48W72W19WOOW04WSOW36W18W69W67W68W27W66W52W14W66W20W34W06W94W28W57WOOW33W84WOOW82W57W82W38W20W64W28W52W20W

ELV

785457350118715071282610

000000000

980937

00000

112217001702564

00000000000000000000

DELAY

0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

DELAY(1)0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.130.00.00.00.01

-0.190.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.14

DELAY(2)0000000000000000000

-0-000000000000000000000000000

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.20

.0

.0

.0

.02

.23

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.16

DELAY(FINAL)0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.230.00.00.0

-0.03-0.230.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.15

144

4849505152535455565758596061626364656667686970717273747576777879808182838485868788899091929394959697

SCYSBBPVR*GFP*DRP*DHBBOLFRIPYRPECMRDCSLCSP

*RCPORP

*LNA*LCL*FMATCNSCR

*LCMSJR

*JBF*IPCHCC*HCM*BHRVPDTWLTPCTCCSJQTINSWMSYPSPMSCISBCRVRPLMPASMWCLGCISAIRCIKPHAYGSCGLACWC

34 63441304534 7334634 1334436593434335334203414341733463346334733503342335934 634 13337335933583359335934 03348341634 63359333737 3344334313344325834263359332134 8341333503539342332383342351833 33626

.37N

.30N

.50N

.76N

.70N

.05N

.50N

.50N

.08N

.52N

.57N

.94N

.88N

.66N

.72N

.35N

.32N

.92N

.67N

.37N

.07N

.20N

.58N

.24N

.64N

.64N

.51N

.96N

.70N

.35N

.67N

.20N

.30N

.ION

.60N

.80N

.80N

.50N

.60N

.20N

.90N

.43N

.15N

.SON

.40N

.93N

.50N

.ION

.15N

.30N

118271174911821118181182411823118121194211844117 9117141171.11721118 811812118 31181111817118 011827118171175011820118201182211822118211174511835116 2118 011750118131183411958118201183211942117221165111810118 3118 91182811824116 6115381164811449118 4

.25W

.50W

.40W

.59W

.20W

.13W

.27W

.50W

.46W

.64W

.46W68W.45W.OOW.04W.27W.55W.12W.77W.25W.22W.70W.68W.07W.98W.98W.72W.73W.67W.92W.77W.70W.70W.90W.70W.08W.80W.80W.50W.70W.30W.46W.02W.40W.OOW.48W.30W.30W.59W.70W

287850

00000

119124761697514901268

00000000000000

183390720299165

11951220130544521990

26016922951730

178355809574399906271620

0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

-0-00

-0-0-0000000

-00000

-00

-00

-000000

-000000

-00

-000

-00

-0000000000

.19

.11

.0

.10

.77

.06

.0

.0

.26

.03

.0

.0

.12

.02

.0

.11

.04

.09

.02

.18

.10

.05

.04

.05

.0

.12

.10

.12

.27

.0

.01

.01

.0

.17

.0

.02

.06

.35

.01

.15

.10

.11

.05

.58

.09

.0

.0

.0

.0

.0

-0.21-0.150.0

-0.19-0.80-0.070.00.00.31

-0.010.00.0

-0.150.020.00.150.08

-0.120.01

-0.230.16

-0.060.100.110.00.160.16

-0.150.280.0

-0.020.000.0

-0.150.0

-0.040.090.47

-0.020.22

-0.130.110.070.830.100.00.00.00.00.0

-0.23-0.170.0

-0.23-0.85-0.080.00.00.33

-0.050.00.0

-0.170.030.00.160.10-0.130.01

-0.250.20

-0.060.140.150.00.180.19

-0.160.280.0

-0.03-0.020.0

-0.120.0

-0.040.120.48

-0.020.26

-0.140.100.090.970.090.00.00.00.00.0

145

9899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129

CLCCISBARWKRTAYSHGPTVPNCPKFJOLGDHGASSUFPTDKYPSIPSADEGGECFCLPCJPCAMSBSNSBSMSBSCSBLPSBLGSBLCSBCDSBCCSBAIOCB

35493324324035483556362436 63633355236 535493555342434 034 6341234 434 7342734 534103415331434 23359343334 634293422345634 034 2

.OON

.40N

.80N

.87N

.73N

.83N

.50N

.73N

.91N

.02N

.86N

.90N

.58N

.25N

.ION

.26N

.88N

.95N

.48N

.33N

.92N

.27N

.70N

.25N

.68N

.62N

.57N

.79N

.12N

.48N

.79N

.20N

1173511824116401203012028121151204312128120241211012021120201191211848118521184611839119 8119 51185711859119 111930120201193712024119 311942119201201011926119 1

.80W

.20W

.30W

.67W

.45W

.22W

.27W

.18W

.81W

.15W

.17W

.22W

.15W

.37W

.77W

.92W

.90W

.85W

.44W

.85W

.21W

.99W

.40W

.99W

.99W

.03W

.85W

.81W

.63W

.32W

.23W

.OOW

7664855105035521925064034693364331189

041701701727

010055493172712591724571344151190213610104-75

0.00.00.00.0C.O0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

0-0-000000000000

-0-0-000

-0-00

-0000

-000000

.37

.05

.10

.0

.0

.0

.0

.0

.0

.0

.0

.0

.0

.12

.17

.12

.06

.0

.55

.16

.16

.28

.09

.10

.08

.0

.19

.16

.25

.0

.0

.26

0.60-0.050.040.00.00.00.00.00.00.00.00.00.00.10

-0.21-0.14-0.090.00.72

-0.22-0.250.33

-0.080.370.130.0

-0.250.240.380.00.00.31

0.74-0.040.090.00.00.00.00.00.00.00.00.00.00.08

-0.23-0.16-0.120.00.77

-0.26-0.280.34

-0.090.460.210.0

-0.270.260.430.00.00.30

146

TABLE 7. List of seismic stations used for recording and relocating 1976 earthquakes. The last column represents the station delay factor used in the final relocation for 1976 earth­ quakes.

147

STN LAT LONG ELV

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748

BLUSMESOWCKCDB2KEEBC2C02SHHINSRMRBMMCFTTPRSCYSBBGFPDHBPYRPECCSPRCPLNALCLFMATCNSCRLCMSJRIPCHCMBHRVPDTWLTPCTCCSJQSWMSYPSCIRVRPLMPASNWCISAIKPGSCCLC

3424.32N3349 . 36N3426. 55N34 8,18N3 344. ION3338. 30N3339. 42N3350. 83N3411. 26N3356. 14N3412. 77N3345.40N34 2. UN34 5.33N34 6.37N3441.30N34 7.76N34 1.05N3434.08N3353. 52N3417. 88N3346. 66N3347. 35N3350. 32N3342. 9 2N3359. 67N34 6.37N34 1.07N3337. 20N3358. 24N3359. 64N34 0.51N3348. 96N3416.70N34 6.35N3359. 67N3337. 20N344 3. ION3431. 60N3258. 80N3359. 60N3321.20N34 8.90N3413. 43N3539. SON3238. 93N3518. ION3549. OON

11743. 52W11721. 32W117 4.45W11710. 48W117 3.72W11639.19W11527. 67W11520.68W11539.27W11611. 66W11634.52W11435. 14W117 6.66W11835.20W11827. 25W11 749. SOWllklk.59W11823. 13W11844. 46W117 0.64W11721. 45W118 8.00W118 3.27W11811.55W11817. 12W118 0.77W11827. 25W11817. 22W11750.70W11820.07W11822. 98W11821. 72W11745. 73W11835. 67W116 2.92W118 0.77W11750.70W11834.90W11958.70W11832.80W11722.50W11651. 70W11810.30W118 3.46W11828.40W116 6.48W11648.30W11735. 80W

00000000

112217001702564

00

287850

00

12476161268

00000000000

183390720299165

12201305219260

1692295

1730835957990766

DELAY(1)0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

DELAY(2)

-0.11-0.090.00.200.090.00.00.00.00.00.00.00.070.03

-0.030.10

-0.07-0.140.170.08

-0.030.140.050.06

-0.100.12

-0.060.20

-0.050.100.240.88

-0.050.020.00.130.0

-0.150.020.00.00.0-0.130.00.00.00.00.0

DELAYFINAL-0.18-0.120.00.290.080.00.00.00.00.00.00.00.140.01

-0.040.10

-0.11-0.180.220.11

-0.050.220.070.14

-0.120.19

-0.080.36-0.080.170.300.82

-0.080.020.00.170.0

-0.160.050.00.00.0

-0.17-0.020.00.00.00.0

148

4950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798

CISPTDKYPSIPSADECFCAM

SBSNSBSMSBSCSBLPSBLGSBLCSBCDSBCC

IRCPSP1MBABLRYSCRGLHULRRTPOCOYHOTPEMRODSMOFTCDC 5PKMDC1BCHVSTSNSRMRADLLTCCRRCOACOKDAHINGPLTRUNSGLSUPMDATHR

3324. 40N34 0.25N34 6. ION3412. 26N34 4.88N3427. 48N3415.27N3314.70N34 2.25N3359. 68N3433. 62N34 6.57N3429.79N3422. 12N3456. 48N3423. 40N3347. 63N35 5.24N3451. 05N3438.60N3514. 53N3440. 26N3431. 50N3452. 70N3321.84N3318. 84N3410.04N3437. 78N3332. 15N3452. 25N3419.63N35 3.00N3420. 82N3511. ION33 9.40N3325. 90N3412.77N3433. 38N3329. 34N3253. 18N3251. 8 IN3250. 95N3244. 07N3259 . 30N3243.87N3258. 33N3238. 95N3257. 3 IN3354. 78N3433.19N

11824.20W11848. 37W11852. 77W11846. 92W11839.90W119 5.44W119 1.99W11930.40W12020. 99W11937. 99W12024. 03W119 3.85W11942. 81W11920.63W12010. 32W11824.00W11632. 93W11932.08W11913. 25W11921. 10W11943. 40W11824.68W118 1.70W11813. SOW11618. 63W11634.89W11752. 18W11636. 29W11627.70W11853. 51W11937. 99W120 2.40W11935. 09W120 5.05W11713. 90W11732. 90W11634. 52W11725. 02W115 4.20W11558. 10W115 7.36W11543. 61W11533. 47W11518. 61W11443.76W11458.63W11543. 52W11549.43W11659. 97W11743. 10W

48541

701701727

1005271259172457134415

1190213610580195

00000000000

800000000

112919

1702090

4580000

-600000

8451025

0.00.00.00.00.00.00.00.0 .0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.000.00.000.00.000.00.00.00.00.00.00.00.00.00.00.0

0.00.11

-0.12-0.080.010.230.17

-0.080.11

-0.120.0

-0.09-0.010.120.00.040.00.0

-0.070.300.0

-0.080.060.320.00.0

-0.210.00.0

-0.040.290.0

-0.180.00.00

-0.140.000.310.000.00.00.00.00.00.00.00.00.00.450.0

0.000.12

-0.12-0.110.010.310.21

-0.130.16

-0.170.0

-0.11-0.000.190.00.030.00.0

-0.030.320.0

-0.110.030.360.00.0

-0.250.00.0

-0.040.270.0

-0.180.00.00

-0.160.000.390.000.00.00.00.00.00.00.00.00.00.590.0

149

99100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140

SILSSKPCFRAYVRGPICLGAYMDHDGSNRSONYEGCPEZSBCZGSCZRVRZPLMZSCIZSBBZMWCZPASZSYPZTPCZISAZCWCZCLCZCISZGLAZHAYZBARZMLLWWRPNMCPM

CPTZVGRGAVSSVLEDCOQCPEBMT

3420341234 334 2335032543245323334253251324134263252342635183359332132583441341334 8343134 6353936263549332433 33342324034 53359335834 93318335034 1341334283351325235 8

.87N

.97N

.19N

.18N

.25N

.85N

.58N

.28N

.73N

.71N

.67N

.18N

.80N

.50N

.ION

.60N

.20N

.80N

.30N

.40N

.95N

.63N

.45N

.80N

.35N

.OON

.40N

.ION

.40N

.80M

.48N

.51N

.64N

.24N

.20N

.25N

.35N

.02N

.06N

.63N

.80N

.15N

116491174111747116481164811438114291143211618115261151611957117 6119421164811722116511183211749118 31181011958116 211828118 4117351182411449115381164011656116391154811611117201164811730117291155611730117 611835

.60W

.32W

.44W

.67W

.53W

.59W

.57W

.68W

.30W

.21W

.11W

.56W

.OOW

.80W

.30W

.50W

.70W

.80W

.50W

.50W

.29W

.67W

.92W

.40W

.68W

.80W

.20W

.60W

.20W

.30W

.18W

.36W

.05W

.SOW

.40W

.53W

.34W

.34W

.19W

.58W

.OOW

.01W

17301765162

234215002636876

134700000000

2198501730308130576183516207664856274395101513702

114793761

15000

1594853

00

1237

0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

0.00.0

-0.050.00.00.00.00.00.00.00.00.00.590.620.0

-0.060.240.020.0

-0.03-0.15-0.100.00.00.00.0

-0.130.00.00.00.00.00.00.00.050.0

-0.130.00.00.370.00.0

0.00.0

-0.120.00.00.00.00.00.00.00.00.00.630.760.0

-0.090.410.080.02

-0.07-0.18-0.120.00.00.00.0

-0.200.00.00.00.00.00.00.00.100.0

-0.180.00.00.350.00.0

150

APPENDIX B: Improvement in earthquake relocationsby applying three successive iterations of the station delay constant to seismic station data.

Note: Improvement in the quality of earthquake relocations is noted by the shift in the class "B" relocations to class "A" relocations. Quality rating for earthquake relocations is explained in Table 3 which is presented in the text

151

1973-1975 Data

Iteration 1

Quality of relocation: Number of earthquakes; Percentage:

Iteration 2

Quality of relocation: Number of earthquakes: Percentage:

Iteration 3

Quality of relocation: Number of earthquakes: Percentage:

Iteration 1

Quality of relocation: Number of earthquakes: Percentage:

Iteration 2

Quality of relocation: Number of earthquakes; Percentage:

Iteration 3

Quality of relocation: Number of earthquakes: Percentage:

A1

2.4

A5

12.

A7

17.

1976

A2

6.5

A6

19.

A6

19.

B3892.7

B33

8 84.6

B31

9 79.5

Data

B2787.1

B24

4 77.4

B24

4 77.4

C2

4.9

C1

2.6

C1

2.6

C1

3.2

C0

0.0

C0

0.0

D0

0.0

D0

0.0

D0

0.0

D1

3.2

D1

3.2

D1

3.2

Total no. ofEarthquakes

41

Total no. of Earthquakes

39

Total no. of Earthquakes

39

Total no. of Earthquakes

31

Total no. of Earthquakes

31

Total no. of Earthquakes

31

152

APPENDIX C: HYPO71 (REVISED) data output summary for three typical earthquakes.

Note:

1. Iterative step-wise locational proce­ dure

2. Statistical information describing the earthquake relocation

3. Phase-card list

4. First-motion plot of P-wave readings from the phase list.

153

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10

APPENDIX D: Summary list of 423 earthquakes relocated in the study area which have occurred between January 1, 1973 and December 31, 1976.

Note: Table 3 (in text) explains the legend for this output summary list.

160

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170

APPENDIX E: Epicenter, magnitude, and focal depth plots for the study area.

Note: Scale is 1:250,000. Plotting symbols are described in Figures 13, 14, and 15.

171

- zt

NRGNITUDF. PLOT 1973 - 1976

EPICENTER PLOT1973 - 1976

\ "

FOCRL DEPTH °LCT 1973-76 R RN3 B DRTR

APPENDIX F: Epicenter, magnitude, and focal depth plots for Point Mugu area.

Note: Scale is 1:83,333. Plotting symbols described in Figures 13, 14, and 15.

176

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183

APPENDIX G: Problems limiting the precision of a fault-plane solution

184

Three possible problems are capable of limiting the

precision of a fault-plane solution., The first is the

interpretation of the P-wave signal, which is often ill-

defined at its onset in terms of both origin time and

polarity. Presently, the USGS, CIT, and USC all use one

seismogram-record reader to reduce these inconsistencies,

although various readers have been employed since 1973.

Scheduled quarry blasts set off at Corona, Jensen, and

Eagle Mountain in the Mojave Desert, located east of

approximately 118°20'W longitude, send a compressional

blast wavefront throughout southern California which is

regularly recorded at seismic stations. These blasts

served as checks on station polarities.

Second, ambiguity exists in the selection of either

one of the two orthogonal nodal planes as the proper fault

plane for any particular earthquake the alternative nodal

plane becomes the auxiliary fault plane. S-wave data

does not help to distinguish between the two possible

fault planes in the case of a double-couple source

mechanism since the shear-wave radiation pattern is also

composed of four symmetrical lobes. Structural lineaments

and seismicity trends paralleling a nodal plane are

criteria often used to distinguish between the fault plane

and the auxiliary fault plane.

The final problem is the sensitivity of the fault-

plane solution to the crustal velocity model. In order

185

to establish the variance in the fault-plane solution as

a function of the crustal velocity model, the first-motion

plots and corresponding fault-plane solutions for five

well-located earthquakes have been compared with respect

to the six crustal velocity models suggested above for

southern California. For each of these earthquakes,

located in the central Los Angeles basin, a P-wave first-

motion plot has been produced using the six different

crustal velocity models. The poles to each fault plane

have been plotted in this Appendix. The variation in the

orientation of the fault planes due to a changing crustal

velocity model is tabulated in Table 8. The change in

strike and dip for a fault plane, as a function of the

crustal velocity models considered, is 36.5 and 27.0 ,

respectively. Thus, the velocity data is a limiting factor

in the precision of the fault-plane solutions.

186

Variation in fault-plane orientation versus crustal velocity model. Poles to each possible fault plane are plotted for the six crustal velocity models discussed in the text.

U = USGS/Healy modelL = Los Angeles basin modelM - Pt. Mugu modelK = Kanamori and Hadley (Mojave Desert) modelT = Transverse Ranges model

See Table 8.

187

180

180

oo 00

event

740227

event

740306

180

160

even

t 74

0312

even

t 74

1106

00 vo

event 741219

190

TABLE 8

Maximum variation between the strike and dip of possible fault planes due to changes

in crustal velocity models

Event

740277

740306

740311

741106

741219

Strike ( )

69341430104223405350

Dip

14303820343016164032

Quality of Earthquake Location

B

A

A

B

B

Standard deviation in strike = 36.5°; in dip = 27.0(

191